DDD with EF Code First - how to put them together?

Question

I am learning DDD development for few days, and i start to like it.

I (think i) understand the principle of DDD, where your main focus is on business objects, where you have aggregates, aggregates roots, repositories just for aggregates roots and so on.

I am trying to create a simple project where i combine DDD development with Code First approach.

My questions are: (I am using asp.net MVC)

1. DDD Business Objects will be different than Code First objects? Even if they will probably be the same, for example i can have a Product business object which has all the rules and methods, and i can have a Product code first (POCO) object which will just contain the properties i need to save in database.

2. If answer to question 1 is "true", then how do i notify the Product POCO object that a property from business object Product has been changed and i have to update it? I am using an "AutoMapper" or something like this? If the answer is "no", i am completely lost.

Can you show me the most simple (CRUD) example of how can i put those two together?

Thank you

Answer 1

Update I no longer advocate for the use of "domain objects" and instead advocate a use of a messaging-based domain model. See here for an example.

The answer to #1 is it depends. In any enterprise application, you're going to find 2 major categories of stuff in the domain:

Straight CRUD

There's no need for a domain object here because the next state of the object doesn't depend on the previous state of the object. It's all data and no behavior. In this case, it's ok to use the same class (i.e. an EF POCO) everywhere: editing, persisting, displaying.

An example of this is saving a billing address on an order:

public class BillingAddress {
  public Guid OrderId;
  public string StreetLine1;
  // etc.
}

On the other hand, we have...

State Machines

You need to have separate objects for domain behavior and state persistence (and a repository to do the work). The public interface on the domain object should almost always be all void methods and no public getters. An example of this would be order status:

public class Order { // this is the domain object  
  private Guid _id;
  private Status _status;

  // note the behavior here - we throw an exception if it's not a valid state transition
  public void Cancel() {  
    if (_status == Status.Shipped)
      throw new InvalidOperationException("Can't cancel order after shipping.")
    _status = Status.Cancelled;
  }

  // etc...
}

public class Data.Order { // this is the persistence (EF) class
  public Guid Id;
  public Status Status;
}

public interface IOrderRepository {
  // The implementation of this will:
  // 1. Load the EF class if it exists or new it up with the ID if it doesn't
  // 2. Map the domain class to the EF class
  // 3. Save the EF class to the DbContext.
  void Save(Order order); 
}

The answer to #2 is that the DbContext will automatically track changes to EF classes.

Answer 2

The Pluralsight course: Entity Framework in the Enterprise goes into this exact scenario of Domain Driven Design incorporated with EF Code First.

For number 1, I believe you can do it either way. It's just a matter of style. For number 2, the instructor in the video goes through a couple ways to account for this. One way is to have a "State" property on every class that is set on the client-side when modifying a value. The DbContext then knows what changes to persist.

Answer 3

The answer is No. One of the best things about EF code-first is that it fits nicely with DDD since you have to create your business objects by hand so do use your EF models to be equivalent to DDD entities and value objects. No need to add an extra layer of complexity, I don't think DDD recommends that anywhere.

You could even have your entities to implement an IEntity and you value objects to implement IValue, additionally follow the rest of DDD patterns namely Repositories to do the actual communication to the database. More of these ideas you can find this very good sample application in .NET, even though it doesn't use EF code first, it's still very valuable: http://code.google.com/p/ndddsample/

Answer 4

Recently I've done similar project. I was following this tutorial: link And I've done it this way: I've created Blank solution, added projects: Domain, Service and WebUI.

Simply said in domain I've put model (for example classes for EF code first, methods etc.) Service was used for domain to communicate with world (WebUI, MobileUI, other sites etc.) using asp.net webapi WebUi was actually MVC application (but model was in domain so it was mostly VC)

Hope I've helped

Answer 5

Late question on this topic. Reading Josh Kodroff's answer confirms my thoughts about the implementation of a Repository to, for instance, Entity Framework DAL.

You map the domain object to an EF persistance object and let EF handle it when saving. When retrieving, you let EF fetch from database and map it to your domain object(aggregate root) and adds it to your collection.

Is this the correct strategy for repository implementation?