Question

I’m starting a new project and have decided to try to incorporate DDD patterns and also include Linq to Entities. When I look at the EF’s ObjectContext it seems to be performing the functions of both Repository and Unit of Work patterns:

Repository in the sense that the underlying data level interface is abstracted from the entity representation and I can request and save data through the ObjectContext.

Unit Of Work in the sense that I can write all my inserts/updates to the objectContext and execute them all in one shot when I do a SaveChanges().

It seems redundant to put another layer of these patterns on top of the EF ObjectContext? It also seems that the Model classes can be incorporated directly on top of the EF generated entities using ‘partial class’.

I’m new at DDD so please let me know if I’m missing something here.

Answer 1

I don’t think that the Entity Framework is a good implementation of Repository, because:

• The object context is insufficiently abstract to do good unit testing of things which reference it, since it is bound to the DB access. Having an IRepository reference instead works much better for creating unit tests.
• When a client has access to the ObjectContext, the client can do pretty much anything it cares to. The only real control you have over this at all is to make certain types or properties private. It is hard to implement good data security this way.
• On a non-trivial model, the ObjectContext is insufficiently abstract. You may, for example, have both tables and stored procedures mapped to the same entity type. You don’t really want the client to have to distinguish between the two mappings.
• On a related note, it is difficult to write comprehensive and well-enforce business rules and entity code. Indeed, whether or not it this is even a good idea is debatable.

On the other hand, once you have an ObjectContext, implementing the Repository pattern is trivial. Indeed, for cases that are not particularly complex, the Repository is something of a wrapper around the ObjectContext and the Entity types.