Multiple table inheritance in Rails

mti.rb

# Multiple table inheritance in Rails
# from http://web.archive.org/web/20120406010350/http://mediumexposure.com/multiple-table-inheritance-active-record
# the below code creates a macro 'acts_as_title' that you can include in an AR model
# including 'acts_as_title' adds the attributes from the TitleProperties model/table
# so that, e.g. a TvTitle can have attributes that are a combination 
# of the tv_titles table and the title_properties table
# and behaves more or less as a normal AR object.
# 
# yeah this looks a little nuts and needs refactoring

#config/initializers/acts_as_title.rb
class ActiveRecord::Base
  def self.acts_as_title
    include ActsAsTitle
  end
end

#app/models/title_properties
class TitleProperties < ActiveRecord::Base
  self.table_name = 'titles'
  belongs_to :acts_as_title, :polymorphic => true, :dependent => :destroy
end
class AddTitleTypeToTitleTable < ActiveRecord::Migration
  def change
    add_column :titles, :title_type, :string, :null => false
    add_column :titles, :title_id, :integer, :null => false
    add_index :titles, [:title_id, :title_type]
  end
end

#app/models/tv_title.rb
class TvTitle < ActiveRecord::Base
  acts_as_title
end

#app/models/acts_as_title.rb
module ActsAsTitle
  def self.included(base)
    base.has_one :title_properties, :as => :title, :autosave => true, :dependent => :destroy
    base.validate :title_properties_must_be_valid
    base.alias_method_chain :title_properties, :autobuild
    base.extend ClassMethods
    base.define_title_properties_accessors
    base.define_nested_attributes_accessors
    base.define_title_properties_associations_accessors
    base.mix_in_title_properties_attributes
  end

  def title_properties_with_autobuild
    title_properties_without_autobuild || build_title_properties
  end

  def method_missing(meth, *args, &blk)
    title_properties.send(meth, *args, &blk)
  rescue NoMethodError
    super
  end

  module ClassMethods
    def define_title_properties_accessors
      all_attributes = TitleProperties.content_columns.map(&:name)
      ignored_attributes = ["created_at", "updated_at", "title_type"]
      attributes_to_delegate = all_attributes - ignored_attributes
      attributes_to_delegate.each do |attrib|
        class_eval <<-RUBY
          def #{attrib}
            title_properties.#{attrib}
          end

          def #{attrib}=(value)
            self.title_properties.#{attrib} = value
          end

          def #{attrib}?
            self.title_properties.#{attrib}?
          end
        RUBY
      end
    end
    def define_nested_attributes_accessors
      TitleProperties.nested_attributes_options.keys.each do |attrib|
        module_eval <<-RUBY
          delegate :#{attrib}_attributes=, :to => :title_properties, :allow_nil => true
        RUBY
      end
    end
    def define_title_properties_associations_accessors
      associations = TitleProperties.reflect_on_all_associations
      associations.map(&:name).each do |attrib|
        class_eval <<-RUBY
          def #{attrib}
            title_properties.#{attrib}
          end

          def #{attrib}=(value)
            self.title_properties.#{attrib} = value
          end

          def #{attrib}?
            self.title_properties.#{attrib}?
          end
        RUBY
      end
      TitleProperties.generated_feature_methods.public_instance_methods(false).grep(/_ids?\Z/).each do |attrib|
        class_eval <<-RUBY
          def #{attrib}
            self.title_properties.#{attrib}
          end
          def #{attrib}=(value)
            self.title_properties.#{attrib} = value
          end
        RUBY
      end
      instance_eval <<-RUBY
        def reflect_on_all_associations
          associations = TitleProperties.reflect_on_all_associations
          super + associations
        end
        def reflect_on_association(assoc)
          TitleProperties.reflect_on_association(assoc) || super
        end
      RUBY
    end
    def mix_in_title_properties_attributes
      class_eval <<-RUBY
        def column_for_attribute(attrib)
          super || title_properties.column_for_attribute(attrib)
        end
        def attributes
          super.merge(:title_properties => title_properties.attributes)
        end
      RUBY
    end

  end

protected
  def title_properties_must_be_valid
    unless title_properties.valid?
      title_properties.errors.each do |attr, message|
        errors.add(attr, message)
      end
    end
  end
end


#app/inputs/fake_input.rb
# from https://github.com/plataformatec/simple_form/wiki/Create-an-%22input%22-just-for-displaying-attribute-value
# useful in adding values to a simple_form that it doesn't understand, but I do :)
class FakeInput < SimpleForm::Inputs::StringInput
  # This method only create a basic input without reading any value from object
  def input
    template.text_field_tag(attribute_name, nil, input_html_options)
  end
end