# 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