# File lib/dm-core/associations/relationship.rb, line 235
      def parent_key
        return @parent_key if defined?(@parent_key)

        repository_name = parent_repository_name || child_repository_name
        properties      = parent_model.properties(repository_name)

        @parent_key = if @parent_properties
          parent_key = properties.values_at(*@parent_properties)
          properties.class.new(parent_key).freeze
        else
          properties.key
        end
      end