child_key (DataMapper::Associations::Relationship)

    # File lib/dm-core/associations/relationship.rb, line 182
          def child_key
            return @child_key if defined?(@child_key)

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

            @child_key = if @child_properties
              child_key = properties.values_at(*@child_properties)
              properties.class.new(child_key).freeze
            else
              properties.key
            end
          end