# File lib/dm-core/model.rb, line 209
    def self.extended(descendant)
      descendants << descendant

      descendant.instance_variable_set(:@valid,         false)
      descendant.instance_variable_set(:@base_model,    descendant)
      descendant.instance_variable_set(:@storage_names, {})
      descendant.instance_variable_set(:@default_order, {})

      descendant.extend(Chainable)

      extra_extensions.each { |mod| descendant.extend(mod)         }
      extra_inclusions.each { |mod| descendant.send(:include, mod) }
    end