parent_key (DataMapper::Associations::Relationship)

    # 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