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 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
# File lib/active_record/inheritance.rb, line 223 def initialize_dup(other) super ensure_proper_type end
Private Instance Methods
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 240 def ensure_proper_type klass = self.class if klass.finder_needs_type_condition? write_attribute(klass.inheritance_column, klass.sti_name) end end
# File lib/active_record/inheritance.rb, line 230 def initialize_internals_callback super ensure_proper_type end