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I have an ASP.NET MVC 2 application with a POCO domain model and an NHibernate repository layer. My domain model has no awareness of my viewmodels so I use automapper to go from viewmodel to entity and vice/versa.
When I introduced WCF to my project (a late requirement), I started having to deal with disconnected objects. That is, I retrieve an entity from the database with NHibernate and once that entity is serialized it becomes disconnected and each child collection is loaded regardless of whether or not I plan on using it meaning I'm doing alot of unnecessary database work.
After reading up on this, I see that it is highly recommended that you not expose your entities outside of your domain project and you should instead use DTOs.
I see the reason for this but I'm having trouble figuring out how to implement it.
Do I map from viewmodel to DTO in ASP.NET MVC, send DTOs through the service layer, and map from DTO to entity in the service layer? Where should I define my DTOs?
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Data Transfer Objects (DTOs) and View Models (VMs) are not the same concept! The main difference is that while VMs can encapsulate behaviour, DTOs do not. The purpose of a DTO is the transfer of data from one part of an application to another.
If using anemic data model (i.e. your domain objects don't have any logic), DTO and domain object can be the same object. No. Domain objects have no specific relation to any persistence. In simple words, they are parts to ensure the business logic required to run the application.
In the field of programming a data transfer object (DTO) is an object that carries data between processes. The motivation for its use is that communication between processes is usually done resorting to remote interfaces (e.g., web services), where each call is an expensive operation.
Difference between DTO & Entity: Entity is class mapped to table. Dto is class mapped to "view" layer mostly. What needed to store is entity & which needed to 'show' on web page is DTO.
I like to have my service layer keep entities encapsulated within it, and return/receive only DTOs. I keep the service contracts as well as the DTO's in a separate assembly which both the MVC project and the Service implementation reference.
Inside the service call implementation, the service maps dto's to entities, then does the interaction with repositories and other entities as it needs to.
On the app/mvc project I sometimes will get lazy and just use DTO's as the models for certain actions (especially CRUDy ones). If i need a projection or something like that, then I'll make a viewmodel and convert between DTO and viewmodel with automapper etc.
How exposed your entities are is a subject of much debate. Some people will push them all the way to the view/app layer. I prefer to keep them in the service layer. I find that when the entities leave the service layer, you find yourself doing business logic type stuff anywhere where they're interacted with, stuff that should probably reside in a service.
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