updates to entity,domain driven design

Question

Lets say I have an order which got updated on the UI with some values (they could be ok/not ok to ensure save)
1. How do we validate the changes made? Should the DTO which carries the order back to service layer be validated for completeness?
Once the validation is complete? How does the service return the validation errors? Do we compose a ReponseDTO object and return it like

ResponseDTO saveOrder(OrderDTO);

1. How do we update the domain entity order? Should the DTO Assembler take care of updating the order entity with the latest changes?

If we imagine a typical tiered' approach, ASP .NET on Web Server, WCF on Application Server. When the Order form is updated with data on the web and saved. The WCF receives a OrderDTO. Now how do we update the order from DTO? Do we use an assembler to update the domain object with changes from DTO? something like

class OrderDTOAssembler {
 updateDomainObject(Order, OrderDTO)
}

Answer 1

I will try answer some of your questions from my experience and how I should approach your problem.

First I should not let DTO conduct any validations, but just plain POCO DTO's usually have different properties with specific datatypes, so some kind of validation is done. I mean you have to apply an integer street number and string for street name etc.

Second as you point out. Let a ORderDTOAssembler convert from OrderDTO to Order and vice versa. This is done in the application layer.

Third I would use Visitor pattern Validation in a Domain Driven Design like the example. The OrderService will use an IOrderRepository to save/update the order. But using the visitor-validation approach the OrderService vill call Order.ValidatePersistance (see link in example - but this is a extension method that is implemented in infrastructure layer since it has "db knowledge") to check its state is valid. If true, then we to IOrderRepository.Save(order).

at last, if Order.ValidatePersistance fails we get one or more BrokenRules messages. These should be returned to client in a ResponseDTO. Then cient can act on messages and take action. Problem here can be that you will have a ResponseOrderDTO messages but maybe (just came up with this now) all your ResponseDTO can inherit from ResponseBaseDTO class that expose necessary properties for delivering BrokenRule messages.

I hope you find my thoughts useful and good luck.