Inheritable settings between two models, in Rails

child.rb

class Child < ActiveRecord::Base
  belongs_to :parent

  INHERITABLE_SETTINGS = %i[foo bar baz]

  INHERITABLE_SETTINGS.each do |attr|
    define_method attr do
      value     = read_attribute(attr)
      default   = default_setting(attr)
      inherited = inherited_setting(attr)

      inherited && value == default ? inherited : value
    end

    # This is a total hack. It won't respond to `includes` or `joins` correctly.
    # If we want better support, we'll have to monkeypatch `add_constraints` in Rails 4.1
    # or `last_chain_scope` in >= 4.2 on ActiveRecord::Associations::AssociationScope.
    if /(?<association>.+)_id/ =~ attr
      define_method association do
        assoc = association(association)
        assoc.association_scope.bind_values = [[
          assoc.klass.columns_hash['id'], public_send(attr)
        ]]
        assoc.load_target
      end
    end
  end

  def inherited_setting(attr)
    return unless parent
    parent.settings[attr.to_s]
  end

  def default_setting(attr)
    self.class.column_defaults[attr.to_s]
  end
end

parent.rb

# Allows the child's attributes to be read and written by a regular form.
class Parent < ActiveRecord::Base
  Child::INHERITABLE_SETTINGS.each do |attr|
    define_method attr do
      settings[attr.to_s]
    end

    define_method "#{attr}=" do |value|
      settings_will_change!

      if [nil, ''].include? value
        settings.delete attr.to_s
      else
        settings[attr.to_s] = cast_property_value(attr, value)
      end
    end
  end

  def cast_child_value(attr, value)
    case Child.columns_hash[attr.to_s].type
    when :boolean
      ActiveRecord::ConnectionAdapters::Column.value_to_boolean value
    when :integer
      value.to_i
    when :decimal
      value.to_f
    else
      value
    end
  end
end

rails_monkeypatch.rb

# The orignal version would prevent inheritable settings from working with associations.
# Specifically, it's used in `find_target?`, which decides whether to do a database request.

module SupportInhertiableAssociations
  def foreign_key_present?
    owner.public_send reflection.foreign_key
  end
end

ActiveRecord::Associations::BelongsToAssociation.prepend SupportInhertiableAssociations