# module ActiveRecord::Inheritance

## Single table inheritance¶ ↑

Active Record allows inheritance by storing the name of the class in a column that by default is named “type” (can be changed by overwriting `Base.inheritance_column`). This means that an inheritance looking like this:

    class Company < ActiveRecord::Base; end
    class Firm < Company; end
    class Client < Company; end
    class PriorityClient < Client; end

When you do `Firm.create(name: "37signals")`, this record will be saved in the companies table with type = “Firm”. You can then fetch this row again using `Company.where(name: '37signals').first` and it will return a Firm object.

Be aware that because the type column is an attribute on the record every new subclass will instantly be marked as dirty and the type column will be included in the list of changed attributes on the record. This is different from non Single Table Inheritance(STI) classes:

    Company.new.changed? # => false
    Firm.new.changed?    # => true
    Firm.new.changes     # => {"type"=>["","Firm"]}

If you don't have a type column defined in your table, single-table inheritance won't be triggered. In that case, it'll work just like normal subclasses with no special magic for differentiating between them or reloading the right type with find.

Note, all the attributes for all the cases are kept in the same table. Read more: www.martinfowler.com/eaaCatalog/singleTableInheritance.html

### Public Instance Methods

initialize_dup(other) click to toggle source
Calls superclass method

    # File lib/active_record/inheritance.rb, line 229
    def initialize_dup(other)
      super
      ensure_proper_type
    end

### Private Instance Methods

ensure_proper_type() click to toggle source

Sets the attribute used for single table inheritance to this class name if this is not the ActiveRecord::Base descendant. Considering the hierarchy Reply < Message < ActiveRecord::Base, this makes it possible to do Reply.new without having to set `Reply[Reply.inheritance_column] = "Reply"` yourself. No such attribute would be set for objects of the Message class in that example.

    # File lib/active_record/inheritance.rb, line 246
    def ensure_proper_type
      klass = self.class
      if klass.finder_needs_type_condition?
        write_attribute(klass.inheritance_column, klass.sti_name)
      end
    end

initialize_internals_callback() click to toggle source
Calls superclass method

    # File lib/active_record/inheritance.rb, line 236
    def initialize_internals_callback
      super
      ensure_proper_type
    end