# module ActiveRecord::Associations::ClassMethods

Associations are a set of macro-like class methods for tying objects together through foreign keys. They express relationships like “Project has one Project Manager” or “Project belongs to a Portfolio”. Each macro adds a number of methods to the class which are specialized according to the collection or association symbol and the options hash. It works much the same way as Ruby's own `attr*` methods.

    class Project < ActiveRecord::Base
      belongs_to              :portfolio
      has_one                 :project_manager
      has_many                :milestones
      has_and_belongs_to_many :categories
    end

The project class now has the following methods (and more) to ease the traversal and manipulation of its relationships:

- `Project#portfolio, Project#portfolio=(portfolio), Project#portfolio.nil?`

- `Project#project_manager, Project#project_manager=(project_manager), Project#project_manager.nil?,`

- `Project#milestones.empty?, Project#milestones.size, Project#milestones, Project#milestones<<(milestone),` `Project#milestones.delete(milestone), Project#milestones.destroy(milestone), Project#milestones.find(milestone_id),` `Project#milestones.build, Project#milestones.create`

- `Project#categories.empty?, Project#categories.size, Project#categories, Project#categories<<(category1),` `Project#categories.delete(category1), Project#categories.destroy(category1)`

### A word of warning¶ ↑

Don't create associations that have the same name as instance methods of `ActiveRecord::Base`. Since the association adds a method with that name to its model, using an association with the same name as one provided by `ActiveRecord::Base` will override the method inherited through `ActiveRecord::Base` and will break things. For instance, `attributes` and `connection` would be bad choices for association names, because those names already exist in the list of `ActiveRecord::Base` instance methods.

## Auto-generated methods¶ ↑

See also Instance Public methods below for more details.

### Singular associations (one-to-one)¶ ↑

                                      |            |  belongs_to  |
    generated methods                 | belongs_to | :polymorphic | has_one
    ----------------------------------+------------+--------------+---------
    other                             |     X      |      X       |    X
    other=(other)                     |     X      |      X       |    X
    build_other(attributes={})        |     X      |              |    X
    create_other(attributes={})       |     X      |              |    X
    create_other!(attributes={})      |     X      |              |    X
    reload_other                      |     X      |      X       |    X

### Collection associations (one-to-many / many-to-many)¶ ↑

                                      |       |          | has_many
    generated methods                 | habtm | has_many | :through
    ----------------------------------+-------+----------+----------
    others                            |   X   |    X     |    X
    others=(other,other,...)          |   X   |    X     |    X
    other_ids                         |   X   |    X     |    X
    other_ids=(id,id,...)             |   X   |    X     |    X
    others<<                          |   X   |    X     |    X
    others.push                       |   X   |    X     |    X
    others.concat                     |   X   |    X     |    X
    others.build(attributes={})       |   X   |    X     |    X
    others.create(attributes={})      |   X   |    X     |    X
    others.create!(attributes={})     |   X   |    X     |    X
    others.size                       |   X   |    X     |    X
    others.length                     |   X   |    X     |    X
    others.count                      |   X   |    X     |    X
    others.sum(*args)                 |   X   |    X     |    X
    others.empty?                     |   X   |    X     |    X
    others.clear                      |   X   |    X     |    X
    others.delete(other,other,...)    |   X   |    X     |    X
    others.delete_all                 |   X   |    X     |    X
    others.destroy(other,other,...)   |   X   |    X     |    X
    others.destroy_all                |   X   |    X     |    X
    others.find(*args)                |   X   |    X     |    X
    others.exists?                    |   X   |    X     |    X
    others.distinct                   |   X   |    X     |    X
    others.reset                      |   X   |    X     |    X
    others.reload                     |   X   |    X     |    X

### Overriding generated methods¶ ↑

Association methods are generated in a module included into the model class, making overrides easy. The original generated method can thus be called with `super`:

    class Car < ActiveRecord::Base
      belongs_to :owner
      belongs_to :old_owner

      def owner=(new_owner)
        self.old_owner = self.owner
        super
      end
    end

The association methods module is included immediately after the generated attributes methods module, meaning an association will override the methods for an attribute with the same name.

## Cardinality and associations¶ ↑

Active Record associations can be used to describe one-to-one, one-to-many and many-to-many relationships between models. Each model uses an association to describe its role in the relation. The belongs_to association is always used in the model that has the foreign key.

### One-to-one¶ ↑

Use has_one in the base, and belongs_to in the associated model.

    class Employee < ActiveRecord::Base
      has_one :office
    end
    class Office < ActiveRecord::Base
      belongs_to :employee    # foreign key - employee_id
    end

### One-to-many¶ ↑

Use has_many in the base, and belongs_to in the associated model.

    class Manager < ActiveRecord::Base
      has_many :employees
    end
    class Employee < ActiveRecord::Base
      belongs_to :manager     # foreign key - manager_id
    end

### Many-to-many¶ ↑

There are two ways to build a many-to-many relationship.

The first way uses a has_many association with the `:through` option and a join model, so there are two stages of associations.

    class Assignment < ActiveRecord::Base
      belongs_to :programmer  # foreign key - programmer_id
      belongs_to :project     # foreign key - project_id
    end
    class Programmer < ActiveRecord::Base
      has_many :assignments
      has_many :projects, through: :assignments
    end
    class Project < ActiveRecord::Base
      has_many :assignments
      has_many :programmers, through: :assignments
    end

For the second way, use has_and_belongs_to_many in both models. This requires a join table that has no corresponding model or primary key.

    class Programmer < ActiveRecord::Base
      has_and_belongs_to_many :projects       # foreign keys in the join table
    end
    class Project < ActiveRecord::Base
      has_and_belongs_to_many :programmers    # foreign keys in the join table
    end

Choosing which way to build a many-to-many relationship is not always simple. If you need to work with the relationship model as its own entity, use has_many `:through`. Use has_and_belongs_to_many when working with legacy schemas or when you never work directly with the relationship itself.

## Is it a belongs_to or has_one association?¶ ↑

Both express a 1-1 relationship. The difference is mostly where to place the foreign key, which goes on the table for the class declaring the belongs_to relationship.

    class User < ActiveRecord::Base
      # I reference an account.
      belongs_to :account
    end

    class Account < ActiveRecord::Base
      # One user references me.
      has_one :user
    end

The tables for these classes could look something like:

    CREATE TABLE users (
      id int NOT NULL auto_increment,
      account_id int default NULL,
      name varchar default NULL,
      PRIMARY KEY  (id)
    )

    CREATE TABLE accounts (
      id int NOT NULL auto_increment,
      name varchar default NULL,
      PRIMARY KEY  (id)
    )

## Unsaved objects and associations¶ ↑

You can manipulate objects and associations before they are saved to the database, but there is some special behavior you should be aware of, mostly involving the saving of associated objects.

You can set the `:autosave` option on a has_one, belongs_to, has_many, or has_and_belongs_to_many association. Setting it to `true` will always save the members, whereas setting it to `false` will never save the members. More details about `:autosave` option is available at AutosaveAssociation.

### One-to-one associations¶ ↑

- Assigning an object to a has_one association automatically saves that object and the object being replaced (if there is one), in order to update their foreign keys - except if the parent object is unsaved (`new_record? == true`).

- If either of these saves fail (due to one of the objects being invalid), an ActiveRecord::RecordNotSaved exception is raised and the assignment is cancelled.

- If you wish to assign an object to a has_one association without saving it, use the `#build_association` method (documented below). The object being replaced will still be saved to update its foreign key.

- Assigning an object to a belongs_to association does not save the object, since the foreign key field belongs on the parent. It does not save the parent either.

### Collections¶ ↑

- Adding an object to a collection (#has_many or has_and_belongs_to_many) automatically saves that object, except if the parent object (the owner of the collection) is not yet stored in the database.

- If saving any of the objects being added to a collection (via `push` or similar) fails, then `push` returns `false`.

- If saving fails while replacing the collection (via `association=`), an ActiveRecord::RecordNotSaved exception is raised and the assignment is cancelled.

- You can add an object to a collection without automatically saving it by using the `collection.build` method (documented below).

- All unsaved (`new_record? == true`) members of the collection are automatically saved when the parent is saved.

## Customizing the query¶ ↑

Associations are built from `Relation` objects, and you can use the Relation syntax to customize them. For example, to add a condition:

    class Blog < ActiveRecord::Base
      has_many :published_posts, -> { where(published: true) }, class_name: 'Post'
    end

Inside the `-> { ... }` block you can use all of the usual Relation methods.

### Accessing the owner object¶ ↑

Sometimes it is useful to have access to the owner object when building the query. The owner is passed as a parameter to the block. For example, the following association would find all events that occur on the user's birthday:

    class User < ActiveRecord::Base
      has_many :birthday_events, ->(user) { where(starts_on: user.birthday) }, class_name: 'Event'
    end

Note: Joining, eager loading and preloading of these associations is not fully possible. These operations happen before instance creation and the scope will be called with a `nil` argument. This can lead to unexpected behavior and is deprecated.

## Association callbacks¶ ↑

Similar to the normal callbacks that hook into the life cycle of an Active Record object, you can also define callbacks that get triggered when you add an object to or remove an object from an association collection.

    class Project
      has_and_belongs_to_many :developers, after_add: :evaluate_velocity

      def evaluate_velocity(developer)
        ...
      end
    end

It's possible to stack callbacks by passing them as an array. Example:

    class Project
      has_and_belongs_to_many :developers,
                              after_add: [:evaluate_velocity, Proc.new { |p, d| p.shipping_date = Time.now}]
    end

Possible callbacks are: `before_add`, `after_add`, `before_remove` and `after_remove`.

If any of the `before_add` callbacks throw an exception, the object will not be added to the collection.

Similarly, if any of the `before_remove` callbacks throw an exception, the object will not be removed from the collection.

## Association extensions¶ ↑

The proxy objects that control the access to associations can be extended through anonymous modules. This is especially beneficial for adding new finders, creators, and other factory-type methods that are only used as part of this association.

    class Account < ActiveRecord::Base
      has_many :people do
        def find_or_create_by_name(name)
          first_name, last_name = name.split(" ", 2)
          find_or_create_by(first_name: first_name, last_name: last_name)
        end
      end
    end

    person = Account.first.people.find_or_create_by_name("David Heinemeier Hansson")
    person.first_name # => "David"
    person.last_name  # => "Heinemeier Hansson"

If you need to share the same extensions between many associations, you can use a named extension module.

    module FindOrCreateByNameExtension
      def find_or_create_by_name(name)
        first_name, last_name = name.split(" ", 2)
        find_or_create_by(first_name: first_name, last_name: last_name)
      end
    end

    class Account < ActiveRecord::Base
      has_many :people, -> { extending FindOrCreateByNameExtension }
    end

    class Company < ActiveRecord::Base
      has_many :people, -> { extending FindOrCreateByNameExtension }
    end

Some extensions can only be made to work with knowledge of the association's internals. Extensions can access relevant state using the following methods (where `items` is the name of the association):

- `record.association(:items).owner` - Returns the object the association is part of.

- `record.association(:items).reflection` - Returns the reflection object that describes the association.

- `record.association(:items).target` - Returns the associated object for belongs_to and has_one, or the collection of associated objects for has_many and has_and_belongs_to_many.

However, inside the actual extension code, you will not have access to the `record` as above. In this case, you can access `proxy_association`. For example, `record.association(:items)` and `record.items.proxy_association` will return the same object, allowing you to make calls like `proxy_association.owner` inside association extensions.

## Association Join Models¶ ↑

Has Many associations can be configured with the `:through` option to use an explicit join model to retrieve the data. This operates similarly to a has_and_belongs_to_many association. The advantage is that you're able to add validations, callbacks, and extra attributes on the join model. Consider the following schema:

    class Author < ActiveRecord::Base
      has_many :authorships
      has_many :books, through: :authorships
    end

    class Authorship < ActiveRecord::Base
      belongs_to :author
      belongs_to :book
    end

    @author = Author.first
    @author.authorships.collect { |a| a.book } # selects all books that the author's authorships belong to
    @author.books                              # selects all books by using the Authorship join model

You can also go through a has_many association on the join model:

    class Firm < ActiveRecord::Base
      has_many   :clients
      has_many   :invoices, through: :clients
    end

    class Client < ActiveRecord::Base
      belongs_to :firm
      has_many   :invoices
    end

    class Invoice < ActiveRecord::Base
      belongs_to :client
    end

    @firm = Firm.first
    @firm.clients.flat_map { |c| c.invoices } # select all invoices for all clients of the firm
    @firm.invoices                            # selects all invoices by going through the Client join model

Similarly you can go through a has_one association on the join model:

    class Group < ActiveRecord::Base
      has_many   :users
      has_many   :avatars, through: :users
    end

    class User < ActiveRecord::Base
      belongs_to :group
      has_one    :avatar
    end

    class Avatar < ActiveRecord::Base
      belongs_to :user
    end

    @group = Group.first
    @group.users.collect { |u| u.avatar }.compact # select all avatars for all users in the group
    @group.avatars                                # selects all avatars by going through the User join model.

An important caveat with going through has_one or has_many associations on the join model is that these associations are read-only. For example, the following would not work following the previous example:

    @group.avatars << Avatar.new   # this would work if User belonged_to Avatar rather than the other way around
    @group.avatars.delete(@group.avatars.last)  # so would this

## Setting Inverses¶ ↑

If you are using a belongs_to on the join model, it is a good idea to set the `:inverse_of` option on the belongs_to, which will mean that the following example works correctly (where `tags` is a has_many `:through` association):

    @post = Post.first
    @tag = @post.tags.build name: "ruby"
    @tag.save

The last line ought to save the through record (a `Tagging`). This will only work if the `:inverse_of` is set:

    class Tagging < ActiveRecord::Base
      belongs_to :post
      belongs_to :tag, inverse_of: :taggings
    end

If you do not set the `:inverse_of` record, the association will do its best to match itself up with the correct inverse. Automatic inverse detection only works on has_many, has_one, and belongs_to associations.

Extra options on the associations, as defined in the `AssociationReflection::INVALID_AUTOMATIC_INVERSE_OPTIONS` constant, will also prevent the association's inverse from being found automatically.

The automatic guessing of the inverse association uses a heuristic based on the name of the class, so it may not work for all associations, especially the ones with non-standard names.

You can turn off the automatic detection of inverse associations by setting the `:inverse_of` option to `false` like so:

    class Tagging < ActiveRecord::Base
      belongs_to :tag, inverse_of: false
    end

## Nested Associations¶ ↑

You can actually specify any association with the `:through` option, including an association which has a `:through` option itself. For example:

    class Author < ActiveRecord::Base
      has_many :posts
      has_many :comments, through: :posts
      has_many :commenters, through: :comments
    end

    class Post < ActiveRecord::Base
      has_many :comments
    end

    class Comment < ActiveRecord::Base
      belongs_to :commenter
    end

    @author = Author.first
    @author.commenters # => People who commented on posts written by the author

An equivalent way of setting up this association this would be:

    class Author < ActiveRecord::Base
      has_many :posts
      has_many :commenters, through: :posts
    end

    class Post < ActiveRecord::Base
      has_many :comments
      has_many :commenters, through: :comments
    end

    class Comment < ActiveRecord::Base
      belongs_to :commenter
    end

When using a nested association, you will not be able to modify the association because there is not enough information to know what modification to make. For example, if you tried to add a `Commenter` in the example above, there would be no way to tell how to set up the intermediate `Post` and `Comment` objects.

## Polymorphic Associations¶ ↑

Polymorphic associations on models are not restricted on what types of models they can be associated with. Rather, they specify an interface that a has_many association must adhere to.

    class Asset < ActiveRecord::Base
      belongs_to :attachable, polymorphic: true
    end

    class Post < ActiveRecord::Base
      has_many :assets, as: :attachable         # The :as option specifies the polymorphic interface to use.
    end

    @asset.attachable = @post

This works by using a type column in addition to a foreign key to specify the associated record. In the Asset example, you'd need an `attachable_id` integer column and an `attachable_type` string column.

Using polymorphic associations in combination with single table inheritance (STI) is a little tricky. In order for the associations to work as expected, ensure that you store the base model for the STI models in the type column of the polymorphic association. To continue with the asset example above, suppose there are guest posts and member posts that use the posts table for STI. In this case, there must be a `type` column in the posts table.

Note: The `attachable_type=` method is being called when assigning an `attachable`. The `class_name` of the `attachable` is passed as a String.

    class Asset < ActiveRecord::Base
      belongs_to :attachable, polymorphic: true

      def attachable_type=(class_name)
         super(class_name.constantize.base_class.to_s)
      end
    end

    class Post < ActiveRecord::Base
      # because we store "Post" in attachable_type now dependent: :destroy will work
      has_many :assets, as: :attachable, dependent: :destroy
    end

    class GuestPost < Post
    end

    class MemberPost < Post
    end

## Caching¶ ↑

All of the methods are built on a simple caching principle that will keep the result of the last query around unless specifically instructed not to. The cache is even shared across methods to make it even cheaper to use the macro-added methods without worrying too much about performance at the first go.

    project.milestones             # fetches milestones from the database
    project.milestones.size        # uses the milestone cache
    project.milestones.empty?      # uses the milestone cache
    project.milestones(true).size  # fetches milestones from the database
    project.milestones             # uses the milestone cache

## Eager loading of associations¶ ↑

Eager loading is a way to find objects of a certain class and a number of named associations. It is one of the easiest ways to prevent the dreaded N+1 problem in which fetching 100 posts that each need to display their author triggers 101 database queries. Through the use of eager loading, the number of queries will be reduced from 101 to 2.

    class Post < ActiveRecord::Base
      belongs_to :author
      has_many   :comments
    end

Consider the following loop using the class above:

    Post.all.each do |post|
      puts "Post:            " + post.title
      puts "Written by:      " + post.author.name
      puts "Last comment on: " + post.comments.first.created_on
    end

To iterate over these one hundred posts, we'll generate 201 database queries. Let's first just optimize it for retrieving the author:

    Post.includes(:author).each do |post|

This references the name of the belongs_to association that also used the `:author` symbol. After loading the posts, `find` will collect the `author_id` from each one and load all of the referenced authors with one query. Doing so will cut down the number of queries from 201 to 102.

We can improve upon the situation further by referencing both associations in the finder with:

    Post.includes(:author, :comments).each do |post|

This will load all comments with a single query. This reduces the total number of queries to 3. In general, the number of queries will be 1 plus the number of associations named (except if some of the associations are polymorphic belongs_to - see below).

To include a deep hierarchy of associations, use a hash:

    Post.includes(:author, { comments: { author: :gravatar } }).each do |post|

The above code will load all the comments and all of their associated authors and gravatars. You can mix and match any combination of symbols, arrays, and hashes to retrieve the associations you want to load.

All of this power shouldn't fool you into thinking that you can pull out huge amounts of data with no performance penalty just because you've reduced the number of queries. The database still needs to send all the data to Active Record and it still needs to be processed. So it's no catch-all for performance problems, but it's a great way to cut down on the number of queries in a situation as the one described above.

Since only one table is loaded at a time, conditions or orders cannot reference tables other than the main one. If this is the case, Active Record falls back to the previously used `LEFT OUTER JOIN` based strategy. For example:

    Post.includes([:author, :comments]).where(['comments.approved = ?', true])

This will result in a single SQL query with joins along the lines of: `LEFT OUTER JOIN comments ON comments.post_id = posts.id` and `LEFT OUTER JOIN authors ON authors.id = posts.author_id`. Note that using conditions like this can have unintended consequences. In the above example, posts with no approved comments are not returned at all because the conditions apply to the SQL statement as a whole and not just to the association.

You must disambiguate column references for this fallback to happen, for example `order: "author.name DESC"` will work but `order: "name DESC"` will not.

If you want to load all posts (including posts with no approved comments), then write your own `LEFT OUTER JOIN` query using `ON`:

    Post.joins("LEFT OUTER JOIN comments ON comments.post_id = posts.id AND comments.approved = '1'")

In this case, it is usually more natural to include an association which has conditions defined on it:

    class Post < ActiveRecord::Base
      has_many :approved_comments, -> { where(approved: true) }, class_name: 'Comment'
    end

    Post.includes(:approved_comments)

This will load posts and eager load the `approved_comments` association, which contains only those comments that have been approved.

If you eager load an association with a specified `:limit` option, it will be ignored, returning all the associated objects:

    class Picture < ActiveRecord::Base
      has_many :most_recent_comments, -> { order('id DESC').limit(10) }, class_name: 'Comment'
    end

    Picture.includes(:most_recent_comments).first.most_recent_comments # => returns all associated comments.

Eager loading is supported with polymorphic associations.

    class Address < ActiveRecord::Base
      belongs_to :addressable, polymorphic: true
    end

A call that tries to eager load the addressable model

    Address.includes(:addressable)

This will execute one query to load the addresses and load the addressables with one query per addressable type. For example, if all the addressables are either of class Person or Company, then a total of 3 queries will be executed. The list of addressable types to load is determined on the back of the addresses loaded. This is not supported if Active Record has to fallback to the previous implementation of eager loading and will raise ActiveRecord::EagerLoadPolymorphicError. The reason is that the parent model's type is a column value so its corresponding table name cannot be put in the `FROM`/`JOIN` clauses of that query.

## Table Aliasing¶ ↑

Active Record uses table aliasing in the case that a table is referenced multiple times in a join. If a table is referenced only once, the standard table name is used. The second time, the table is aliased as `#{reflection_name}_#{parent_table_name}`. Indexes are appended for any more successive uses of the table name.

    Post.joins(:comments)
    # => SELECT ... FROM posts INNER JOIN comments ON ...
    Post.joins(:special_comments) # STI
    # => SELECT ... FROM posts INNER JOIN comments ON ... AND comments.type = 'SpecialComment'
    Post.joins(:comments, :special_comments) # special_comments is the reflection name, posts is the parent table name
    # => SELECT ... FROM posts INNER JOIN comments ON ... INNER JOIN comments special_comments_posts

Acts as tree example:

    TreeMixin.joins(:children)
    # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
    TreeMixin.joins(children: :parent)
    # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
                                INNER JOIN parents_mixins ...
    TreeMixin.joins(children: {parent: :children})
    # => SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
                                INNER JOIN parents_mixins ...
                                INNER JOIN mixins childrens_mixins_2

Has and Belongs to Many join tables use the same idea, but add a `_join` suffix:

    Post.joins(:categories)
    # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
    Post.joins(categories: :posts)
    # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
                               INNER JOIN categories_posts posts_categories_join INNER JOIN posts posts_categories
    Post.joins(categories: {posts: :categories})
    # => SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
                               INNER JOIN categories_posts posts_categories_join INNER JOIN posts posts_categories
                               INNER JOIN categories_posts categories_posts_join INNER JOIN categories categories_posts_2

If you wish to specify your own custom joins using ActiveRecord::QueryMethods#joins method, those table names will take precedence over the eager associations:

    Post.joins(:comments).joins("inner join comments ...")
    # => SELECT ... FROM posts INNER JOIN comments_posts ON ... INNER JOIN comments ...
    Post.joins(:comments, :special_comments).joins("inner join comments ...")
    # => SELECT ... FROM posts INNER JOIN comments comments_posts ON ...
                               INNER JOIN comments special_comments_posts ...
                               INNER JOIN comments ...

Table aliases are automatically truncated according to the maximum length of table identifiers according to the specific database.

## Modules¶ ↑

By default, associations will look for objects within the current module scope. Consider:

    module MyApplication
      module Business
        class Firm < ActiveRecord::Base
          has_many :clients
        end

        class Client < ActiveRecord::Base; end
      end
    end

When `Firm#clients` is called, it will in turn call `MyApplication::Business::Client.find_all_by_firm_id(firm.id)`. If you want to associate with a class in another module scope, this can be done by specifying the complete class name.

    module MyApplication
      module Business
        class Firm < ActiveRecord::Base; end
      end

      module Billing
        class Account < ActiveRecord::Base
          belongs_to :firm, class_name: "MyApplication::Business::Firm"
        end
      end
    end

## Bi-directional associations¶ ↑

When you specify an association, there is usually an association on the associated model that specifies the same relationship in reverse. For example, with the following models:

    class Dungeon < ActiveRecord::Base
      has_many :traps
      has_one :evil_wizard
    end

    class Trap < ActiveRecord::Base
      belongs_to :dungeon
    end

    class EvilWizard < ActiveRecord::Base
      belongs_to :dungeon
    end

The `traps` association on `Dungeon` and the `dungeon` association on `Trap` are the inverse of each other, and the inverse of the `dungeon` association on `EvilWizard` is the `evil_wizard` association on `Dungeon` (and vice-versa). By default, Active Record can guess the inverse of the association based on the name of the class. The result is the following:

    d = Dungeon.first
    t = d.traps.first
    d.object_id == t.dungeon.object_id # => true

The `Dungeon` instances `d` and `t.dungeon` in the above example refer to the same in-memory instance since the association matches the name of the class. The result would be the same if we added `:inverse_of` to our model definitions:

    class Dungeon < ActiveRecord::Base
      has_many :traps, inverse_of: :dungeon
      has_one :evil_wizard, inverse_of: :dungeon
    end

    class Trap < ActiveRecord::Base
      belongs_to :dungeon, inverse_of: :traps
    end

    class EvilWizard < ActiveRecord::Base
      belongs_to :dungeon, inverse_of: :evil_wizard
    end

There are limitations to `:inverse_of` support:

- does not work with `:through` associations.

- does not work with `:polymorphic` associations.

- inverse associations for belongs_to associations has_many are ignored.

For more information, see the documentation for the `:inverse_of` option.

## Deleting from associations¶ ↑

### Dependent associations¶ ↑

has_many, has_one, and belongs_to associations support the `:dependent` option. This allows you to specify that associated records should be deleted when the owner is deleted.

For example:

    class Author
      has_many :posts, dependent: :destroy
    end
    Author.find(1).destroy # => Will destroy all of the author's posts, too

The `:dependent` option can have different values which specify how the deletion is done. For more information, see the documentation for this option on the different specific association types. When no option is given, the behavior is to do nothing with the associated records when destroying a record.

Note that `:dependent` is implemented using Rails' callback system, which works by processing callbacks in order. Therefore, other callbacks declared either before or after the `:dependent` option can affect what it does.

Note that `:dependent` option is ignored for has_one `:through` associations.

### Delete or destroy?¶ ↑

has_many and has_and_belongs_to_many associations have the methods `destroy`, `delete`, `destroy_all` and `delete_all`.

For has_and_belongs_to_many, `delete` and `destroy` are the same: they cause the records in the join table to be removed.

For has_many, `destroy` and `destroy_all` will always call the `destroy` method of the record(s) being removed so that callbacks are run. However `delete` and `delete_all` will either do the deletion according to the strategy specified by the `:dependent` option, or if no `:dependent` option is given, then it will follow the default strategy. The default strategy is to do nothing (leave the foreign keys with the parent ids set), except for has_many `:through`, where the default strategy is `delete_all` (delete the join records, without running their callbacks).

There is also a `clear` method which is the same as `delete_all`, except that it returns the association rather than the records which have been deleted.

### What gets deleted?¶ ↑

There is a potential pitfall here: has_and_belongs_to_many and has_many `:through` associations have records in join tables, as well as the associated records. So when we call one of these deletion methods, what exactly should be deleted?

The answer is that it is assumed that deletion on an association is about removing the link between the owner and the associated object(s), rather than necessarily the associated objects themselves. So with has_and_belongs_to_many and has_many `:through`, the join records will be deleted, but the associated records won't.

This makes sense if you think about it: if you were to call `post.tags.delete(Tag.find_by(name: 'food'))` you would want the 'food' tag to be unlinked from the post, rather than for the tag itself to be removed from the database.

However, there are examples where this strategy doesn't make sense. For example, suppose a person has many projects, and each project has many tasks. If we deleted one of a person's tasks, we would probably not want the project to be deleted. In this scenario, the delete method won't actually work: it can only be used if the association on the join model is a belongs_to. In other situations you are expected to perform operations directly on either the associated records or the `:through` association.

With a regular has_many there is no distinction between the “associated records” and the “link”, so there is only one choice for what gets deleted.

With has_and_belongs_to_many and has_many `:through`, if you want to delete the associated records themselves, you can always do something along the lines of `person.tasks.each(&:destroy)`.

## Type safety with ActiveRecord::AssociationTypeMismatch¶ ↑

If you attempt to assign an object to an association that doesn't match the inferred or specified `:class_name`, you'll get an ActiveRecord::AssociationTypeMismatch.

## Options¶ ↑

All of the association macros can be specialized through options. This makes cases more complex than the simple and guessable ones possible.

### Public Instance Methods

belongs_to(name, scope = nil, options = {}) click to toggle source

Specifies a one-to-one association with another class. This method should only be used if this class contains the foreign key. If the other class contains the foreign key, then you should use has_one instead. See also ActiveRecord::Associations::ClassMethods's overview on when to use has_one and when to use belongs_to.

Methods will be added for retrieval and query for a single associated object, for which this object holds an id:

`association` is a placeholder for the symbol passed as the `name` argument, so `belongs_to :author` would add among others `author.nil?`.

association

  Returns the associated object. `nil` is returned if none is found.

association=(associate)

  Assigns the associate object, extracts the primary key, and sets it as the foreign key.

build_association(attributes = {})

  Returns a new object of the associated type that has been instantiated with `attributes` and linked to this object through a foreign key, but has not yet been saved.

create_association(attributes = {})

  Returns a new object of the associated type that has been instantiated with `attributes`, linked to this object through a foreign key, and that has already been saved (if it passed the validation).

create_association!(attributes = {})

  Does the same as `create_association`, but raises ActiveRecord::RecordInvalid if the record is invalid.

reload_association

  Returns the associated object, forcing a database read.

### Example¶ ↑

A Post class declares `belongs_to :author`, which will add:

- `Post#author` (similar to `Author.find(author_id)`)

- `Post#author=(author)` (similar to `post.author_id = author.id`)

- `Post#build_author` (similar to `post.author = Author.new`)

- `Post#create_author` (similar to `post.author = Author.new; post.author.save; post.author`)

- `Post#create_author!` (similar to `post.author = Author.new; post.author.save!; post.author`)

- `Post#reload_author`

The declaration can also include an `options` hash to specialize the behavior of the association.

### Scopes¶ ↑

You can pass a second argument `scope` as a callable (i.e. proc or lambda) to retrieve a specific record or customize the generated query when you access the associated object.

Scope examples:

    belongs_to :firm, -> { where(id: 2) }
    belongs_to :user, -> { joins(:friends) }
    belongs_to :level, ->(game) { where("game_level > ?", game.current_level) }

### Options¶ ↑

:class_name

  Specify the class name of the association. Use it only if that name can't be inferred from the association name. So `belongs_to :author` will by default be linked to the Author class, but if the real class name is Person, you'll have to specify it with this option.

:foreign_key

  Specify the foreign key used for the association. By default this is guessed to be the name of the association with an “_id” suffix. So a class that defines a `belongs_to :person` association will use “person_id” as the default `:foreign_key`. Similarly, `belongs_to :favorite_person, class_name: "Person"` will use a foreign key of “favorite_person_id”.

:foreign_type

  Specify the column used to store the associated object's type, if this is a polymorphic association. By default this is guessed to be the name of the association with a “_type” suffix. So a class that defines a `belongs_to :taggable, polymorphic: true` association will use “taggable_type” as the default `:foreign_type`.

:primary_key

  Specify the method that returns the primary key of associated object used for the association. By default this is id.

:dependent

  If set to `:destroy`, the associated object is destroyed when this object is. If set to `:delete`, the associated object is deleted without calling its destroy method. This option should not be specified when belongs_to is used in conjunction with a has_many relationship on another class because of the potential to leave orphaned records behind.

:counter_cache

  Caches the number of belonging objects on the associate class through the use of ActiveRecord::CounterCache::ClassMethods#increment_counter and ActiveRecord::CounterCache::ClassMethods#decrement_counter. The counter cache is incremented when an object of this class is created and decremented when it's destroyed. This requires that a column named `#{table_name}_count` (such as `comments_count` for a belonging Comment class) is used on the associate class (such as a Post class) - that is the migration for `#{table_name}_count` is created on the associate class (such that `Post.comments_count` will return the count cached, see note below). You can also specify a custom counter cache column by providing a column name instead of a `true`/`false` value to this option (e.g., `counter_cache: :my_custom_counter`.) Note: Specifying a counter cache will add it to that model's list of readonly attributes using `attr_readonly`.

:polymorphic

  Specify this association is a polymorphic association by passing `true`. Note: If you've enabled the counter cache, then you may want to add the counter cache attribute to the `attr_readonly` list in the associated classes (e.g. `class Post; attr_readonly :comments_count; end`).

:validate

  When set to `true`, validates new objects added to association when saving the parent object. `false` by default. If you want to ensure associated objects are revalidated on every update, use `validates_associated`.

:autosave

  If true, always save the associated object or destroy it if marked for destruction, when saving the parent object. If false, never save or destroy the associated object. By default, only save the associated object if it's a new record.

  Note that ActiveRecord::NestedAttributes::ClassMethods#accepts_nested_attributes_for sets `:autosave` to `true`.

:touch

  If true, the associated object will be touched (the updated_at/on attributes set to current time) when this record is either saved or destroyed. If you specify a symbol, that attribute will be updated with the current time in addition to the updated_at/on attribute. Please note that with touching no validation is performed and only the `after_touch`, `after_commit` and `after_rollback` callbacks are executed.

:inverse_of

  Specifies the name of the has_one or has_many association on the associated object that is the inverse of this belongs_to association. Does not work in combination with the `:polymorphic` options. See ActiveRecord::Associations::ClassMethods's overview on Bi-directional associations for more detail.

:optional

  When set to `true`, the association will not have its presence validated.

:required

  When set to `true`, the association will also have its presence validated. This will validate the association itself, not the id. You can use `:inverse_of` to avoid an extra query during validation. NOTE: `required` is set to `true` by default and is deprecated. If you don't want to have association presence validated, use `optional: true`.

:default

  Provide a callable (i.e. proc or lambda) to specify that the association should be initialized with a particular record before validation.

Option examples:

    belongs_to :firm, foreign_key: "client_of"
    belongs_to :person, primary_key: "name", foreign_key: "person_name"
    belongs_to :author, class_name: "Person", foreign_key: "author_id"
    belongs_to :valid_coupon, ->(o) { where "discounts > ?", o.payments_count },
                              class_name: "Coupon", foreign_key: "coupon_id"
    belongs_to :attachable, polymorphic: true
    belongs_to :project, -> { readonly }
    belongs_to :post, counter_cache: true
    belongs_to :comment, touch: true
    belongs_to :company, touch: :employees_last_updated_at
    belongs_to :user, optional: true
    belongs_to :account, default: -> { company.account }

    # File lib/active_record/associations.rb, line 1679
    def belongs_to(name, scope = nil, options = {})
      reflection = Builder::BelongsTo.build(self, name, scope, options)
      Reflection.add_reflection self, name, reflection
    end

has_and_belongs_to_many(name, scope = nil, **options, &extension) click to toggle source

Specifies a many-to-many relationship with another class. This associates two classes via an intermediate join table. Unless the join table is explicitly specified as an option, it is guessed using the lexical order of the class names. So a join between Developer and Project will give the default join table name of “developers_projects” because “D” precedes “P” alphabetically. Note that this precedence is calculated using the `<` operator for String. This means that if the strings are of different lengths, and the strings are equal when compared up to the shortest length, then the longer string is considered of higher lexical precedence than the shorter one. For example, one would expect the tables “paper_boxes” and “papers” to generate a join table name of “papers_paper_boxes” because of the length of the name “paper_boxes”, but it in fact generates a join table name of “paper_boxes_papers”. Be aware of this caveat, and use the custom `:join_table` option if you need to. If your tables share a common prefix, it will only appear once at the beginning. For example, the tables “catalog_categories” and “catalog_products” generate a join table name of “catalog_categories_products”.

The join table should not have a primary key or a model associated with it. You must manually generate the join table with a migration such as this:

    class CreateDevelopersProjectsJoinTable < ActiveRecord::Migration[5.0]
      def change
        create_join_table :developers, :projects
      end
    end

It's also a good idea to add indexes to each of those columns to speed up the joins process. However, in MySQL it is advised to add a compound index for both of the columns as MySQL only uses one index per table during the lookup.

Adds the following methods for retrieval and query:

`collection` is a placeholder for the symbol passed as the `name` argument, so `has_and_belongs_to_many :categories` would add among others `categories.empty?`.

collection

  Returns a Relation of all the associated objects. An empty Relation is returned if none are found.

collection<<(object, …)

  Adds one or more objects to the collection by creating associations in the join table (`collection.push` and `collection.concat` are aliases to this method). Note that this operation instantly fires update SQL without waiting for the save or update call on the parent object, unless the parent object is a new record.

collection.delete(object, …)

  Removes one or more objects from the collection by removing their associations from the join table. This does not destroy the objects.

collection.destroy(object, …)

  Removes one or more objects from the collection by running destroy on each association in the join table, overriding any dependent option. This does not destroy the objects.

collection=objects

  Replaces the collection's content by deleting and adding objects as appropriate.

collection_singular_ids

  Returns an array of the associated objects' ids.

collection_singular_ids=ids

  Replace the collection by the objects identified by the primary keys in `ids`.

collection.clear

  Removes every object from the collection. This does not destroy the objects.

collection.empty?

  Returns `true` if there are no associated objects.

collection.size

  Returns the number of associated objects.

collection.find(id)

  Finds an associated object responding to the `id` and that meets the condition that it has to be associated with this object. Uses the same rules as ActiveRecord::FinderMethods#find.

collection.exists?(…)

  Checks whether an associated object with the given conditions exists. Uses the same rules as ActiveRecord::FinderMethods#exists?.

collection.build(attributes = {})

  Returns a new object of the collection type that has been instantiated with `attributes` and linked to this object through the join table, but has not yet been saved.

collection.create(attributes = {})

  Returns a new object of the collection type that has been instantiated with `attributes`, linked to this object through the join table, and that has already been saved (if it passed the validation).

collection.reload

  Returns a Relation of all of the associated objects, forcing a database read. An empty Relation is returned if none are found.

### Example¶ ↑

A Developer class declares `has_and_belongs_to_many :projects`, which will add:

- `Developer#projects`

- `Developer#projects<<`

- `Developer#projects.delete`

- `Developer#projects.destroy`

- `Developer#projects=`

- `Developer#project_ids`

- `Developer#project_ids=`

- `Developer#projects.clear`

- `Developer#projects.empty?`

- `Developer#projects.size`

- `Developer#projects.find(id)`

- `Developer#projects.exists?(...)`

- `Developer#projects.build` (similar to `Project.new("developer_id" => id)`)

- `Developer#projects.create` (similar to `c = Project.new("developer_id" => id); c.save; c`)

- `Developer#projects.reload`

The declaration may include an `options` hash to specialize the behavior of the association.

### Scopes¶ ↑

You can pass a second argument `scope` as a callable (i.e. proc or lambda) to retrieve a specific set of records or customize the generated query when you access the associated collection.

Scope examples:

    has_and_belongs_to_many :projects, -> { includes(:milestones, :manager) }
    has_and_belongs_to_many :categories, ->(post) {
      where("default_category = ?", post.default_category)

### Extensions¶ ↑

The `extension` argument allows you to pass a block into a #has_and_belongs_to_many association. This is useful for adding new finders, creators and other factory-type methods to be used as part of the association.

Extension examples:

    has_and_belongs_to_many :contractors do
      def find_or_create_by_name(name)
        first_name, last_name = name.split(" ", 2)
        find_or_create_by(first_name: first_name, last_name: last_name)
      end
    end

### Options¶ ↑

:class_name

  Specify the class name of the association. Use it only if that name can't be inferred from the association name. So `has_and_belongs_to_many :projects` will by default be linked to the Project class, but if the real class name is SuperProject, you'll have to specify it with this option.

:join_table

  Specify the name of the join table if the default based on lexical order isn't what you want. WARNING: If you're overwriting the table name of either class, the `table_name` method MUST be declared underneath any has_and_belongs_to_many declaration in order to work.

:foreign_key

  Specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and “_id” suffixed. So a Person class that makes a has_and_belongs_to_many association to Project will use “person_id” as the default `:foreign_key`.

:association_foreign_key

  Specify the foreign key used for the association on the receiving side of the association. By default this is guessed to be the name of the associated class in lower-case and “_id” suffixed. So if a Person class makes a has_and_belongs_to_many association to Project, the association will use “project_id” as the default `:association_foreign_key`.

:validate

  When set to `true`, validates new objects added to association when saving the parent object. `true` by default. If you want to ensure associated objects are revalidated on every update, use `validates_associated`.

:autosave

  If true, always save the associated objects or destroy them if marked for destruction, when saving the parent object. If false, never save or destroy the associated objects. By default, only save associated objects that are new records.

  Note that ActiveRecord::NestedAttributes::ClassMethods#accepts_nested_attributes_for sets `:autosave` to `true`.

Option examples:

    has_and_belongs_to_many :projects
    has_and_belongs_to_many :projects, -> { includes(:milestones, :manager) }
    has_and_belongs_to_many :nations, class_name: "Country"
    has_and_belongs_to_many :categories, join_table: "prods_cats"
    has_and_belongs_to_many :categories, -> { readonly }

    # File lib/active_record/associations.rb, line 1844
            def has_and_belongs_to_many(name, scope = nil, **options, &extension)
              habtm_reflection = ActiveRecord::Reflection::HasAndBelongsToManyReflection.new(name, scope, options, self)

              builder = Builder::HasAndBelongsToMany.new name, self, options

              join_model = ActiveSupport::Deprecation.silence { builder.through_model }

              const_set join_model.name, join_model
              private_constant join_model.name

              middle_reflection = builder.middle_reflection join_model

              Builder::HasMany.define_callbacks self, middle_reflection
              Reflection.add_reflection self, middle_reflection.name, middle_reflection
              middle_reflection.parent_reflection = habtm_reflection

              include Module.new {
                class_eval "              def destroy_associations
                    association(:#{middle_reflection.name}).delete_all(:delete_all)
                    association(:#{name}).reset
                    super
                  end
    ", __FILE__, __LINE__ + 1
              }

              hm_options = {}
              hm_options[:through] = middle_reflection.name
              hm_options[:source] = join_model.right_reflection.name

              [:before_add, :after_add, :before_remove, :after_remove, :autosave, :validate, :join_table, :class_name, :extend].each do |k|
                hm_options[k] = options[k] if options.key? k
              end

              ActiveSupport::Deprecation.silence { has_many name, scope, hm_options, &extension }
              _reflections[name.to_s].parent_reflection = habtm_reflection
            end

has_many(name, scope = nil, options = {}, &extension) click to toggle source

Specifies a one-to-many association. The following methods for retrieval and query of collections of associated objects will be added:

`collection` is a placeholder for the symbol passed as the `name` argument, so `has_many :clients` would add among others `clients.empty?`.

collection(force_reload = false)

  Returns a Relation of all the associated objects. An empty Relation is returned if none are found.

collection<<(object, …)

  Adds one or more objects to the collection by setting their foreign keys to the collection's primary key. Note that this operation instantly fires update SQL without waiting for the save or update call on the parent object, unless the parent object is a new record. This will also run validations and callbacks of associated object(s).

collection.delete(object, …)

  Removes one or more objects from the collection by setting their foreign keys to `NULL`. Objects will be in addition destroyed if they're associated with `dependent: :destroy`, and deleted if they're associated with `dependent: :delete_all`.

  If the `:through` option is used, then the join records are deleted (rather than nullified) by default, but you can specify `dependent: :destroy` or `dependent: :nullify` to override this.

collection.destroy(object, …)

  Removes one or more objects from the collection by running `destroy` on each record, regardless of any dependent option, ensuring callbacks are run.

  If the `:through` option is used, then the join records are destroyed instead, not the objects themselves.

collection=objects

  Replaces the collections content by deleting and adding objects as appropriate. If the `:through` option is true callbacks in the join models are triggered except destroy callbacks, since deletion is direct by default. You can specify `dependent: :destroy` or `dependent: :nullify` to override this.

collection_singular_ids

  Returns an array of the associated objects' ids

collection_singular_ids=ids

  Replace the collection with the objects identified by the primary keys in `ids`. This method loads the models and calls `collection=`. See above.

collection.clear

  Removes every object from the collection. This destroys the associated objects if they are associated with `dependent: :destroy`, deletes them directly from the database if `dependent: :delete_all`, otherwise sets their foreign keys to `NULL`. If the `:through` option is true no destroy callbacks are invoked on the join models. Join models are directly deleted.

collection.empty?

  Returns `true` if there are no associated objects.

collection.size

  Returns the number of associated objects.

collection.find(…)

  Finds an associated object according to the same rules as ActiveRecord::FinderMethods#find.

collection.exists?(…)

  Checks whether an associated object with the given conditions exists. Uses the same rules as ActiveRecord::FinderMethods#exists?.

collection.build(attributes = {}, …)

  Returns one or more new objects of the collection type that have been instantiated with `attributes` and linked to this object through a foreign key, but have not yet been saved.

collection.create(attributes = {})

  Returns a new object of the collection type that has been instantiated with `attributes`, linked to this object through a foreign key, and that has already been saved (if it passed the validation). Note: This only works if the base model already exists in the DB, not if it is a new (unsaved) record!

collection.create!(attributes = {})

  Does the same as `collection.create`, but raises ActiveRecord::RecordInvalid if the record is invalid.

collection.reload

  Returns a Relation of all of the associated objects, forcing a database read. An empty Relation is returned if none are found.

### Example¶ ↑

A `Firm` class declares `has_many :clients`, which will add:

- `Firm#clients` (similar to `Client.where(firm_id: id)`)

- `Firm#clients<<`

- `Firm#clients.delete`

- `Firm#clients.destroy`

- `Firm#clients=`

- `Firm#client_ids`

- `Firm#client_ids=`

- `Firm#clients.clear`

- `Firm#clients.empty?` (similar to `firm.clients.size == 0`)

- `Firm#clients.size` (similar to `Client.count "firm_id = #{id}"`)

- `Firm#clients.find` (similar to `Client.where(firm_id: id).find(id)`)

- `Firm#clients.exists?(name: 'ACME')` (similar to `Client.exists?(name: 'ACME', firm_id: firm.id)`)

- `Firm#clients.build` (similar to `Client.new("firm_id" => id)`)

- `Firm#clients.create` (similar to `c = Client.new("firm_id" => id); c.save; c`)

- `Firm#clients.create!` (similar to `c = Client.new("firm_id" => id); c.save!`)

- `Firm#clients.reload`

The declaration can also include an `options` hash to specialize the behavior of the association.

### Scopes¶ ↑

You can pass a second argument `scope` as a callable (i.e. proc or lambda) to retrieve a specific set of records or customize the generated query when you access the associated collection.

Scope examples:

    has_many :comments, -> { where(author_id: 1) }
    has_many :employees, -> { joins(:address) }
    has_many :posts, ->(blog) { where("max_post_length > ?", blog.max_post_length) }

### Extensions¶ ↑

The `extension` argument allows you to pass a block into a #has_many association. This is useful for adding new finders, creators and other factory-type methods to be used as part of the association.

Extension examples:

    has_many :employees do
      def find_or_create_by_name(name)
        first_name, last_name = name.split(" ", 2)
        find_or_create_by(first_name: first_name, last_name: last_name)
      end
    end

### Options¶ ↑

:class_name

  Specify the class name of the association. Use it only if that name can't be inferred from the association name. So `has_many :products` will by default be linked to the `Product` class, but if the real class name is `SpecialProduct`, you'll have to specify it with this option.

:foreign_key

  Specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and “_id” suffixed. So a Person class that makes a has_many association will use “person_id” as the default `:foreign_key`.

:foreign_type

  Specify the column used to store the associated object's type, if this is a polymorphic association. By default this is guessed to be the name of the polymorphic association specified on “as” option with a “_type” suffix. So a class that defines a `has_many :tags, as: :taggable` association will use “taggable_type” as the default `:foreign_type`.

:primary_key

  Specify the name of the column to use as the primary key for the association. By default this is `id`.

:dependent

  Controls what happens to the associated objects when their owner is destroyed. Note that these are implemented as callbacks, and Rails executes callbacks in order. Therefore, other similar callbacks may affect the `:dependent` behavior, and the `:dependent` behavior may affect other callbacks.

  - `:destroy` causes all the associated objects to also be destroyed.

  - `:delete_all` causes all the associated objects to be deleted directly from the database (so callbacks will not be executed).

  - `:nullify` causes the foreign keys to be set to `NULL`. Callbacks are not executed.

  - `:restrict_with_exception` causes an exception to be raised if there are any associated records.

  - `:restrict_with_error` causes an error to be added to the owner if there are any associated objects.

  If using with the `:through` option, the association on the join model must be a belongs_to, and the records which get deleted are the join records, rather than the associated records.

  If using `dependent: :destroy` on a scoped association, only the scoped objects are destroyed. For example, if a Post model defines `has_many :comments, -> { where published: true }, dependent: :destroy` and `destroy` is called on a post, only published comments are destroyed. This means that any unpublished comments in the database would still contain a foreign key pointing to the now deleted post.

:counter_cache

  This option can be used to configure a custom named `:counter_cache.` You only need this option, when you customized the name of your `:counter_cache` on the belongs_to association.

:as

  Specifies a polymorphic interface (See belongs_to).

:through

  Specifies an association through which to perform the query. This can be any other type of association, including other `:through` associations. Options for `:class_name`, `:primary_key` and `:foreign_key` are ignored, as the association uses the source reflection.

  If the association on the join model is a belongs_to, the collection can be modified and the records on the `:through` model will be automatically created and removed as appropriate. Otherwise, the collection is read-only, so you should manipulate the `:through` association directly.

  If you are going to modify the association (rather than just read from it), then it is a good idea to set the `:inverse_of` option on the source association on the join model. This allows associated records to be built which will automatically create the appropriate join model records when they are saved. (See the 'Association Join Models' section above.)

:source

  Specifies the source association name used by has_many `:through` queries. Only use it if the name cannot be inferred from the association. `has_many :subscribers, through: :subscriptions` will look for either `:subscribers` or `:subscriber` on Subscription, unless a `:source` is given.

:source_type

  Specifies type of the source association used by has_many `:through` queries where the source association is a polymorphic belongs_to.

:validate

  When set to `true`, validates new objects added to association when saving the parent object. `true` by default. If you want to ensure associated objects are revalidated on every update, use `validates_associated`.

:autosave

  If true, always save the associated objects or destroy them if marked for destruction, when saving the parent object. If false, never save or destroy the associated objects. By default, only save associated objects that are new records. This option is implemented as a `before_save` callback. Because callbacks are run in the order they are defined, associated objects may need to be explicitly saved in any user-defined `before_save` callbacks.

  Note that ActiveRecord::NestedAttributes::ClassMethods#accepts_nested_attributes_for sets `:autosave` to `true`.

:inverse_of

  Specifies the name of the belongs_to association on the associated object that is the inverse of this has_many association. Does not work in combination with `:through` or `:as` options. See ActiveRecord::Associations::ClassMethods's overview on Bi-directional associations for more detail.

:extend

  Specifies a module or array of modules that will be extended into the association object returned. Useful for defining methods on associations, especially when they should be shared between multiple association objects.

Option examples:

    has_many :comments, -> { order("posted_on") }
    has_many :comments, -> { includes(:author) }
    has_many :people, -> { where(deleted: false).order("name") }, class_name: "Person"
    has_many :tracks, -> { order("position") }, dependent: :destroy
    has_many :comments, dependent: :nullify
    has_many :tags, as: :taggable
    has_many :reports, -> { readonly }
    has_many :subscribers, through: :subscriptions, source: :user

    # File lib/active_record/associations.rb, line 1401
    def has_many(name, scope = nil, options = {}, &extension)
      reflection = Builder::HasMany.build(self, name, scope, options, &extension)
      Reflection.add_reflection self, name, reflection
    end

has_one(name, scope = nil, options = {}) click to toggle source

Specifies a one-to-one association with another class. This method should only be used if the other class contains the foreign key. If the current class contains the foreign key, then you should use belongs_to instead. See also ActiveRecord::Associations::ClassMethods's overview on when to use has_one and when to use belongs_to.

The following methods for retrieval and query of a single associated object will be added:

`association` is a placeholder for the symbol passed as the `name` argument, so `has_one :manager` would add among others `manager.nil?`.

association

  Returns the associated object. `nil` is returned if none is found.

association=(associate)

  Assigns the associate object, extracts the primary key, sets it as the foreign key, and saves the associate object. To avoid database inconsistencies, permanently deletes an existing associated object when assigning a new one, even if the new one isn't saved to database.

build_association(attributes = {})

  Returns a new object of the associated type that has been instantiated with `attributes` and linked to this object through a foreign key, but has not yet been saved.

create_association(attributes = {})

  Returns a new object of the associated type that has been instantiated with `attributes`, linked to this object through a foreign key, and that has already been saved (if it passed the validation).

create_association!(attributes = {})

  Does the same as `create_association`, but raises ActiveRecord::RecordInvalid if the record is invalid.

reload_association

  Returns the associated object, forcing a database read.

### Example¶ ↑

An Account class declares `has_one :beneficiary`, which will add:

- `Account#beneficiary` (similar to `Beneficiary.where(account_id: id).first`)

- `Account#beneficiary=(beneficiary)` (similar to `beneficiary.account_id = account.id; beneficiary.save`)

- `Account#build_beneficiary` (similar to `Beneficiary.new("account_id" => id)`)

- `Account#create_beneficiary` (similar to `b = Beneficiary.new("account_id" => id); b.save; b`)

- `Account#create_beneficiary!` (similar to `b = Beneficiary.new("account_id" => id); b.save!; b`)

- `Account#reload_beneficiary`

### Scopes¶ ↑

You can pass a second argument `scope` as a callable (i.e. proc or lambda) to retrieve a specific record or customize the generated query when you access the associated object.

Scope examples:

    has_one :author, -> { where(comment_id: 1) }
    has_one :employer, -> { joins(:company) }
    has_one :latest_post, ->(blog) { where("created_at > ?", blog.enabled_at) }

### Options¶ ↑

The declaration can also include an `options` hash to specialize the behavior of the association.

Options are:

:class_name

  Specify the class name of the association. Use it only if that name can't be inferred from the association name. So `has_one :manager` will by default be linked to the Manager class, but if the real class name is Person, you'll have to specify it with this option.

:dependent

  Controls what happens to the associated object when its owner is destroyed:

  - `:destroy` causes the associated object to also be destroyed

  - `:delete` causes the associated object to be deleted directly from the database (so callbacks will not execute)

  - `:nullify` causes the foreign key to be set to `NULL`. Callbacks are not executed.

  - `:restrict_with_exception` causes an exception to be raised if there is an associated record

  - `:restrict_with_error` causes an error to be added to the owner if there is an associated object

  Note that `:dependent` option is ignored when using `:through` option.

:foreign_key

  Specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and “_id” suffixed. So a Person class that makes a has_one association will use “person_id” as the default `:foreign_key`.

:foreign_type

  Specify the column used to store the associated object's type, if this is a polymorphic association. By default this is guessed to be the name of the polymorphic association specified on “as” option with a “_type” suffix. So a class that defines a `has_one :tag, as: :taggable` association will use “taggable_type” as the default `:foreign_type`.

:primary_key

  Specify the method that returns the primary key used for the association. By default this is `id`.

:as

  Specifies a polymorphic interface (See belongs_to).

:through

  Specifies a Join Model through which to perform the query. Options for `:class_name`, `:primary_key`, and `:foreign_key` are ignored, as the association uses the source reflection. You can only use a `:through` query through a has_one or belongs_to association on the join model.

:source

  Specifies the source association name used by has_one `:through` queries. Only use it if the name cannot be inferred from the association. `has_one :favorite, through: :favorites` will look for a `:favorite` on Favorite, unless a `:source` is given.

:source_type

  Specifies type of the source association used by has_one `:through` queries where the source association is a polymorphic belongs_to.

:validate

  When set to `true`, validates new objects added to association when saving the parent object. `false` by default. If you want to ensure associated objects are revalidated on every update, use `validates_associated`.

:autosave

  If true, always save the associated object or destroy it if marked for destruction, when saving the parent object. If false, never save or destroy the associated object. By default, only save the associated object if it's a new record.

  Note that ActiveRecord::NestedAttributes::ClassMethods#accepts_nested_attributes_for sets `:autosave` to `true`.

:inverse_of

  Specifies the name of the belongs_to association on the associated object that is the inverse of this has_one association. Does not work in combination with `:through` or `:as` options. See ActiveRecord::Associations::ClassMethods's overview on Bi-directional associations for more detail.

:required

  When set to `true`, the association will also have its presence validated. This will validate the association itself, not the id. You can use `:inverse_of` to avoid an extra query during validation.

Option examples:

    has_one :credit_card, dependent: :destroy  # destroys the associated credit card
    has_one :credit_card, dependent: :nullify  # updates the associated records foreign
                                                  # key value to NULL rather than destroying it
    has_one :last_comment, -> { order('posted_on') }, class_name: "Comment"
    has_one :project_manager, -> { where(role: 'project_manager') }, class_name: "Person"
    has_one :attachment, as: :attachable
    has_one :boss, -> { readonly }
    has_one :club, through: :membership
    has_one :primary_address, -> { where(primary: true) }, through: :addressables, source: :addressable
    has_one :credit_card, required: true

    # File lib/active_record/associations.rb, line 1535
    def has_one(name, scope = nil, options = {})
      reflection = Builder::HasOne.build(self, name, scope, options)
      Reflection.add_reflection self, name, reflection
    end