Using repositories inside a domain entity

If you follow the principles of DDD you'll be well aware of the persistence ignorance discussion\argument. I believe domain entities should be agnostic of the persistence layer and therefore not statically bound at compile time. Overall I'm happy with this approach but it does give issues when trying to place certain business logic on the entity that requires access to some service (read repository).

Now obviously you can use the 'double-dispatch' approach and pass in the repository via an interface and only couple the entity to an interface, but to me this still seems a level of coupling that's unacceptable - usually we see these interfaces in an 'interfaces' project, to couple this to the domain model (entities) seems wrong.

So to get round this you can use functional programming, to be more specific you can use a lambda expression.

So imagine I have a Registration entity and it has responsibility for generating the user name from other data contained in the registration entity. Now to make sure the suggested user name hasn't already been allocated in the database the Registration entity needs to call the database to work this out.

So to avoid coupling the repository interface to the entity we pass in a Func<> into the method and call this to talk to the database, now from the entity perspective it's doesn't know what or how the func is defined it only know the parameters it has to provide and the result from the function - hence reduced coupling.

public sealed class Registration
    {
        public int? Id { get; private set; }
        public string Username { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }

        public Registration(int? id)
        {
            Id = id;
        }

        public void GenerateUserName(Func<string, IList<Registration>> func)
        {
            var suggestedUserName = FirstName + "." + LastName;
            var existingNames = func(suggestedUserName);

            if (existingNames.Count != 0)
                suggestedUserName += (existingNames.Count + 1);

            Username = suggestedUserName;
        }
    }

Now the Func> takes a parameter and returns a list of Registration objects. And you can probably imagine the Func<> is a call to database looking for all matches of the suggested name.

So from the calling perspective the code now looks like this, with a single line to generate the suggested user name and check it against the database.

public sealed class AssetManagementService : IServiceAssetManagement
    {
        public AssetManagementService(IRepository<Registration, string> registrationRepository,
                                      ICanValidate<RegistrationArgs> registrationValidator)
        {
            _registrationRepository = registrationRepository;
        }
  
        public void RegisterUser(Registration registration)
        {
            registration.GenerateUserName(x => _registrationRepository.FindBy(new FindByMatchingUsername(x)));
      
            var result = _registrationValidator.Validate(new RegistrationArgs(registration));
            if (!result.IsValid())
                throw new ArgumentException(result.Message, "registration");

            using (var transaction = new TransactionScope(TransactionScopeOption.Required))
            {
                _registrationRepository.Save(registration);

                transaction.Complete();
            }
        }
    }

Now from an OO perspective this is bad, violations of basic principles of Encapsulation & Abstraction, but and it's a big but it is an attempt to become more declarative and it's definitely using a Functional programming style which whether you like it or not will become a standard addition to any developer toolbox in the future. Personally I starting like the declarative nature and the clean code this produces.

Awkward Coder