assert_valid_fields_with_operator (DataMapper::Aggregates::Query)

    # File lib/dm-aggregates/query.rb, line 13
          def assert_valid_fields_with_operator(fields, unique)
            operators, fields = fields.partition { |f| f.kind_of?(DataMapper::Query::Operator) }

            operators.each do |operator|
              target = operator.target

              unless target == :all || target.kind_of?(Property) && target.model == model && @properties.include?(target)
                raise ArgumentError, "+options[:fields]+ entry #{target.inspect} does not map to a property in #{model}"
              end
            end

            assert_valid_fields_without_operator(fields, unique)
          end