Question

I m trying to use xVal to validate the registration of a new user. I ve ran into a buzz saw when trying to implement the logic that checks to see if the user name that the new user is trying to register with is already taken. I can t seem to find a way to accomplish this without having my User entity have a dependence on my UsersRepository. Here s the method I need to find a way to implement in the my User Entity:

public IEnumerable<ErrorInfo> ValidateUniqueUserName(string username)
{
    if(usersRepository.Users.Exists(m => (m.UserName == username)))
        yield return new ErrorInfo("UserName", "User name already exists");
}

Any ideas on how I can continue to use xVal for this scenario and keep my User entity decoupled from my UsersRepository?

Answer 1

DDD would suggest that you might have a Domain Service to abstract the Users repo from the domain validation (invariant enforcing).

I d like to know where the code from your example above resides (validation service)? But i d suggest you make sure it is in the domain. Here is a clever way to attach complex validation to entities that still support the IDataErrorInfo interface.

What i would suggest is an Domain Service within your validation method that returns your Users.Exists query. Something like:

        base.AddRule(new ValidationRule()
        {
            Properties = "username",
            Description = "User name already exists",
            validator = () => !(new UserService()).Users.Exists(m => (m.UserName == username))
        });

In the example above i will be using DI to inject the appropriate dependencies into the UserService for accessing of the repo/data but you could use a factory or manual DI object creation method if you like:

        base.AddRule(new ValidationRule()
        {
            Properties = "username",
            Description = "User name already exists",
            validator = () =>
                {
                    UserService us = ObjectFactory.GetInstance<UserService>();
                    return !us.Users.Exists(m => (m.UserName == username));
                }
        });

NOTE: The above method requires the validator property to be set to false to indicate an invalid state (in case that wasn t clear).

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