def assert_valid_order(order, fields)
      Array(order).each do |order_entry|
        case order_entry
          when Symbol, String
            unless @properties.named?(order_entry)
              raise ArgumentError, "+options[:order]+ entry #{order_entry.inspect} does not map to a property in #{model}"
            end
        end
      end
    end
def assert_valid_order(order, fields)
      Array(order).each do |order_entry|
        case order_entry
          when Symbol, String
            unless @properties.named?(order_entry)
              raise ArgumentError, "+options[:order]+ entry #{order_entry.inspect} does not map to a property in #{model}"
            end
        end
      end
    end
Verifies that :order option uses proper operator and refers
 to existing property

 @api private