Specify type when creating a record in AR STI

create_with_subtype.rb

#
# This was causing problems when submitting the nested object form
# in a/v/product/channels_edit because all the nested channels would
# be created at plan Channel objects instead of the PrimaryChannel, EusChannel etc.
#
# In active record normally :type is ignored, eg:
#   Channel.create(:type='PrimaryChannel', :etc=>1, ...)
# ignores the type attr and uses 'Channel'.
#
# This hack will make it actually use the :type value.
#
# Usage:
#
#   class User < ActiveRecord::Base
#     extend ActiveRecord::CreateWithSubtype
#       ...
#   end
#
#   class AdminUser < User
#     ...
#   end
#
# >> User.create(:type=>'AdminUser', :username=>'simon', ...) #=> #<AdminUser id:123 ...>
#
# Actually needed this for nested objects in forms using accepts_nested_attributes_for.
#  http://railscasts.com/episodes/197-nested-model-form-part-2
#
# When updating the type is preserved, but when creating a new subobject with the nested form
# the type is always the base class, which in my case was a problem.
#
# (I put this file in lib/active_record/create_with_subtype.rb)
#
module ActiveRecord
  module CreateWithSubtype
    def create(attributes=nil, &block)
      # Check if :type attribute exists. Remove it from attributes
      # (Note that attributes can be an array, hence Hash test)
      specified_type = attributes.delete(:type) if attributes.is_a?(Hash)
      if specified_type
        # Find the specified class
        use_sub_class = const_get(specified_type)
        # Sanity check
        raise "'#{use_sub_class}' not a subclass of '#{self}'" unless self.descendants.include?(use_sub_class)
        # Call create on the specified subclass
        use_sub_class.create(attributes, &block)
      else
        # Otherwise, just do the normal thing
        super(attributes, &block)
      end
    end
  end
end