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Module: ActiveRecord::Inheritance

Relationships & Source Files
Namespace Children
Modules:
ClassMethods
Extension / Inclusion / Inheritance Descendants
Included In:
Base
Super Chains via Extension / Inclusion / Inheritance
Class Chain:
self, ::ActiveSupport::Concern
Defined in: activerecord/lib/active_record/inheritance.rb

Overview

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:

Class Method Summary

::ActiveSupport::Concern - Extended

class_methods

Define class methods from given block.
included

Evaluate given block in context of base class, so that you can write class macros here.
prepended

Evaluate given block in context of base class, so that you can write class macros here.
append_features, prepend_features

Instance Method Summary

DSL Calls

included

[ GitHub ] 


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# File 'activerecord/lib/active_record/inheritance.rb', line 42

included do
  class_attribute :store_full_class_name, instance_writer: false, default: true

  # Determines whether to store the full constant name including namespace when using STI.
  # This is true, by default.
  class_attribute :store_full_sti_class, instance_writer: false, default: true

  set_base_class
end

Instance Method Details

#ensure_proper_type (private)

Sets the attribute used for single table inheritance to this class name if this is not the Base descendant. Considering the hierarchy Reply < Message < 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.

[ GitHub ] 

  


# File 'activerecord/lib/active_record/inheritance.rb', line 359



def ensure_proper_type
  klass = self.class
  if klass.finder_needs_type_condition?
    _write_attribute(klass.inheritance_column, klass.sti_name)
  end
end


  








  

    #initialize_dup(other)  
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/inheritance.rb', line 343



def initialize_dup(other)
  super
  ensure_proper_type
end


  








  

    #initialize_internals_callback   (private)
  

  [ GitHub ]


  

  


# File 'activerecord/lib/active_record/inheritance.rb', line 349



def initialize_internals_callback
  super
  ensure_proper_type
end