child_key (DataMapper::Associations::ManyToMany::Relationship)

    # File lib/dm-core/associations/many_to_many.rb, line 15
            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