Question

This is my first attempt, after reading a lot about DDD, TDD and Repository / UnitOfWork paterns, to make my own application.

I’m using Entity Framework, MVC 4 on .NET 4.0 (the server that will be running this application is a Windows 2003)

This is the basic simplified pattern logic (The original one uses a IRepository, IUnitOfWork, GenericRepository and extends the EF POCOs with a IEntity interface to give access to the common ID field. But this simplified example will be enough to ask my question)

View -> ViewModel -> Controller <- UnitOfWork <- Repository <- EntityFramework <- Database

View

Model.Employee.GetSeniority()

EmployeeDetailsViewModel

Employee e { get; set; }

Employee

DateTime dateHired { get; set; }
TimeSpan GetSeniority()
{
    return DateTime.Today - dateHired;
}

Controller EmployeeDetails()

using(var unitOfWork = new UnitOfWork) {
    return View(EmployeeDetailsViewModel model = new EmployeeDetailsViewModel {
        e = unitOfWork.GetEmployeRepository().Find(o=>o.id == id)
    });
}

UnitOfWork GetEmployeRepository()

return (_employeeRepository ?? _employeeRepository = new EmployeeRepository(this.dbContext));

Repository Find()

dbContext.Configuration.EnableProxyCreation = false;
Employee e = dbContext.Employees.Where(expression);
dbContext.Configuration.EnableProxyCreation = true;
return e;

Everything actually works properly. The problem is that I feel like something is terribly wrong here, and I’m not sure at which layer it should be fixed.

After getting suggested, by a lot of people (Hi Darin), to always pass ViewModels to views and no models, I started doing this. However, what I’m doing (I think) isn’t much better. I’m simply encapsulating my model in a viewmodel. At first, it didn’t sound that bad since my Find() method would turn proxy off before getting an object, which would result in a persistance-ignorant POCO. However, now that I want to add some logic in the POCOs, I feel like there is something wrong.

I think the problem resides in where my Business logic is and the fact that my Employee POCO should be mapped to a DTO object. However, where should I transfer the Employee POCO to a EmployeeDTO? Should that be the task of the Repository, the Controller or something else? I’m also unsure of where I should put my business logic (as simple as GetSeniority() displayed in the example). Should that be added to the EF POCO through partial classes or should that be in the DTOs? Or is there another missing step in the Employee -> EmployeeDTO transfer?

Answer 1

This is a great question. It looks like you are trying to find clean separation which is fantastic. I would break the problem up. You have Data Access and you have UI display and in between you have your business logic. If you want to use a Domain model approach here is how I would structure it.

• Never expose EntityFramework Entity classes outside of Repository. You can choose to return Dto’s (POCO’) or Domain objects from your repository. If you want Dto’s for more separation, that is fine you will just need another layer such as a service layer to convert the Dto’s into Domain Objects.

• Put your business logic in your Domain Objects. So Domain.Employee.GetSenority() would be on your domain object.

• Any logic that does not fit in your Domain Objects can reside in your UnitOfWork or Service Layer.

• Convert Domain objects to ViewModel in controller. At this point map Employee.GetSenority() to MyViewModel.Senority property. Basically your ViewModel is a Dto and only contains view specific logic which typically is not much.

• Where do you call the repository. You could use the UnitOfWork pattern as you have it or simply create a Service Layer class. The key here is that these should be useable to other application types. For instance if you were to write a desktop or Windows 8 Style App, you would likely want to reuse either of these along with your Domain Entities.

I’m sure you are having fun with this. Good luck.