DDD and Entity Framework classes

Consider this simple example:

Domain Model

public class Customer
{
    public Customer(IRegistrar registrar)
    {
        this.registrar = registrar;
    }

    public int Age
    {
        get
        {
            // Just for this example. This will not work for all locals etc but beyond the point here.
            var today = DateTime.Today;
            return today.Year - this.DateOfBirth.Year;
        }
    }

    public DateTime DateOfBirth { get; set; }

    public int Register()
    {
        if (this.Age < 18)
        {
            throw new InvalidOperationException("You must be at least 18 years old");
        }

        int id = this.registrar.Register(this);

        return id;
    }
}

public interface IRegistrar 
{
    public int Register(Customer customer);
}

A lot of people when they do not have a domain model will do this in an MVC controller:

public ActionResult Search(Customer customer)
{
    var today = DateTime.Today;
    var age = today.Year - this.DateOfBirth.Year;
    if (age < 18)
    {
        // Return an error page or the same page but with error etc.
    }

    // All is good
    int id = this.registrar.Register(customer);

    // The rest of code
}

There are a few issues with that:

1. What if the developer forgets to make the check for age before calling registrar? Many people will say, well that is a bad developer. Well whatever the case is, this type of code is prone to bugs.

2. The product is doing well so CFO decides to open up the API because there are many developers out there who are making great UI interfaces for customer registration and they want to use our API. So the developers go ahead and create a WCF service like this:

   public int Register(Customer customer)
   {
       var today = DateTime.Today;
       var age = today.Year - this.DateOfBirth.Year;
       if (age < 18)
       {
           // Return a SOAP fault or some other error
       }
   
       int id = this.registrar.Register(customer);
   
       // The rest of code
   }  
   

3. Now the developers can forget to make the check for age in 2 different places.

4. The code is also in 2 different places. If there is a bug, we need to remember to fix it in 2 different places.
5. If the company starts operating in places where the legal age is 21, we need to find all the places and add this rule.
6. If we are discussing the rules with BA, well we need to look through all the applications and find the rules.

In the above case we only have one rule: Age must be greater than 18. What if we had many more rules and many more classes? You can see where this will go.

EF Model

Your EF model may be like this:

public class Customer
{
    public int Id { get; set; }
    public DateTime DateOfBirth { get; set; }  

    // It may have a foreign key etc.    
}

Application Layer Model

And your model for MVC view maybe like this:

public class Customer
{
    // Or instead of Domain.Customer, it may be a CustomerDto which is used
    // to transfer data from one layer or tier to another.
    // But you get the point.
    public Customer(Domain.Customer customer)
    {
        this.DateOfBirth = customer.DateOfBirth;
        this.Age = customer.Age;
        if (this.DateOfBirth.DayOfYear == DateTime.Today.DayOfYear)
        {
            this.Greeting = "Happy Birthday!!!";
        }
    }
    public int Age { get; set; }

    [Required(ErrorMessage = "Date of birth is required.")]
    [Display(Name = "Data of birth")]
    public DateTime DateOfBirth { get; set; }

    public string Greeting { get; set; }
}

Here is a question: How many EF models have you seen with the Display attribute? I will let you decide if the EF model should concern itself with how it is displayed in the UI. Just the assumption that my EF model will be displayed in UI is wrong. Maybe the only consumers of my class is another web service. I don't think Display should be in the EF model but some may not agree with me; you make the call.

There are loads of questions on stackoverflow about people asking that sometime PropertyX is required and sometimes it is not, how can I do this? Well if you did not put Required attribute on your EF model and use your EF model in your view, then you would not have this issue. There will be one model for the view where PropertyX is a required field. That model will decorate PropertyX with the Required attribute, while another model for the view that does not require PropertyX will not decorate the property with the Required attribute.

ViewModels

And then you may have a viewmodel for a customer for a WPF application and you may have a javascript viewmodel for the frontend (KnockoutJS viewmodel).

Conclusion and answer to your question

So in conclusion, you can have different domain models than your entity models. Your domain model should be unaware of the database. If you decide to remove a column from one table due to normalization and put it into a table of its own, your entity model will be affected. Your domain model should not be affected.

I have read arguments on the net such as "this design takes too long, I just want to roll something out quickly and give it to the client and get paid". Well if you are not designing a product which will need to be maintained and features will be added to it but you are just designing a quick little site for your client then do not use this approach. No design applies to every situation. The point to take away is that your design should be chosen wisely with future in mind.

Also the conversion from entity model to domain to a model for MVC does not need to be done manually. There are libraries out there which will do this for you easily such as AutoMapper.

But I have to admit, there are tons of examples on the net and also in use in many applications where the entity models are used throughout the application and rules are implemented everywhere with loads of if statements.