Skip to main content

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

Comments

Popular posts from this blog

Implementing a busy indicator using a visual overlay in MVVM

This is a technique we use at work to lock the UI whilst some long running process is happening - preventing the user clicking on stuff whilst it's retrieving or rendering data. Now we could have done this by launching a child dialog window but that feels rather out of date and clumsy, we wanted a more modern pattern similar to the way <div> overlays are done on the web. Imagine we have the following simple WPF app and when 'Click' is pressed a busy waiting overlay is shown for the duration entered into the text box. What I'm interested in here is not the actual UI element of the busy indicator but how I go about getting this to show & hide from when using MVVM. The actual UI elements are the standard Busy Indicator coming from the WPF Toolkit : The XAML behind this window is very simple, the important part is the ViewHost. As you can see the ViewHost uses a ContentPresenter element which is bound to the view model, IMainViewModel, it contains 3 child v

Showing a message box from a ViewModel in MVVM

I was doing a code review with a client last week for a WPF app using MVVM and they asked ' How can I show a message from the ViewModel? '. What follows is how I would (and have) solved the problem in the past. When I hear the words ' show a message... ' I instantly think you mean show a transient modal message box that requires the user input before continuing ' with something else ' - once the user has interacted with the message box it will disappear. The following solution only applies to this scenario. The first solution is the easiest but is very wrong from a separation perspective. It violates the ideas behind the Model-View-Controller pattern because it places View concerns inside the ViewModel - the ViewModel now knows about the type of the View and specifically it knows how to show a message box window: The second approach addresses this concern by introducing the idea of messaging\events between the ViewModel and the View. In the example below

Custom AuthorizationHandler for SignalR Hubs

How to implement IAuthorizationRequirement for SignalR in Asp.Net Core v5.0 Been battling this for a couple of days, and eventually ended up raising an issue on Asp.Net Core gitHub  to find the answer. Wanting to do some custom authorization on a SignalR Hub when the client makes a connection (Hub is created) and when an endpoint (Hub method) is called:  I was assuming I could use the same Policy for both class & method attributes, but it ain't so - not because you can't, because you need the signatures to be different. Method implementation has a resource type of HubInnovationContext: I assumed class implementation would have a resource type of HubConnectionContext - client connects etc... This isn't the case, it's infact of type DefaultHttpContext . For me I don't even need that, it can be removed completely  from the inheritence signature and override implementation. Only other thing to note, and this could be a biggy, is the ordering of the statements in th