I want to share an approach commented about and briefly discussed but show an actual example that I am currently using to help unit test EF-based services.
First, I would love to use the in-memory provider from EF Core, but this is about EF 6. Furthermore, for other storage systems like RavenDB, I'd also be a proponent of testing via the in-memory database provider. Again--this is specifically to help test EF-based code without a lot of ceremony.
Here are the goals I had when coming up with a pattern:
- It must be simple for other developers on the team to understand
- It must isolate the EF code at the barest possible level
- It must not involve creating weird multi-responsibility interfaces (such as a "generic" or "typical" repository pattern)
- It must be easy to configure and setup in a unit test
I agree with previous statements that EF is still an implementation detail and it's okay to feel like you need to abstract it in order to do a "pure" unit test. I also agree that ideally, I would want to ensure the EF code itself works--but this involves a sandbox database, in-memory provider, etc. My approach solves both problems--you can safely unit test EF-dependent code and create integration tests to test your EF code specifically.
The way I achieved this was through simply encapsulating EF code into dedicated Query and Command classes. The idea is simple: just wrap any EF code in a class and depend on an interface in the classes that would've originally used it. The main issue I needed to solve was to avoid adding numerous dependencies to classes and setting up a lot of code in my tests.
This is where a useful, simple library comes in: Mediatr. It allows for simple in-process messaging and it does it by decoupling "requests" from the handlers that implement the code. This has an added benefit of decoupling the "what" from the "how". For example, by encapsulating the EF code into small chunks it allows you to replace the implementations with another provider or totally different mechanism, because all you are doing is sending a request to perform an action.
Utilizing dependency injection (with or without a framework--your preference), we can easily mock the mediator and control the request/response mechanisms to enable unit testing EF code.
First, let's say we have a service that has business logic we need to test:
public class FeatureService {
private readonly IMediator _mediator;
public FeatureService(IMediator mediator) {
_mediator = mediator;
}
public async Task ComplexBusinessLogic() {
// retrieve relevant objects
var results = await _mediator.Send(new GetRelevantDbObjectsQuery());
// normally, this would have looked like...
// var results = _myDbContext.DbObjects.Where(x => foo).ToList();
// perform business logic
// ...
}
}
Do you start to see the benefit of this approach? Not only are you explicitly encapsulating all EF-related code into descriptive classes, you are allowing extensibility by removing the implementation concern of "how" this request is handled--this class doesn't care if the relevant objects come from EF, MongoDB, or a text file.
Now for the request and handler, via MediatR:
public class GetRelevantDbObjectsQuery : IRequest<DbObject[]> {
// no input needed for this particular request,
// but you would simply add plain properties here if needed
}
public class GetRelevantDbObjectsEFQueryHandler : IRequestHandler<GetRelevantDbObjectsQuery, DbObject[]> {
private readonly IDbContext _db;
public GetRelevantDbObjectsEFQueryHandler(IDbContext db) {
_db = db;
}
public DbObject[] Handle(GetRelevantDbObjectsQuery message) {
return _db.DbObjects.Where(foo => bar).ToList();
}
}
As you can see, the abstraction is simple and encapsulated. It's also absolutely testable because in an integration test, you could test this class individually--there are no business concerns mixed in here.
So what does a unit test of our feature service look like? It's way simple. In this case, I'm using Moq to do mocking (use whatever makes you happy):
[TestClass]
public class FeatureServiceTests {
// mock of Mediator to handle request/responses
private Mock<IMediator> _mediator;
// subject under test
private FeatureService _sut;
[TestInitialize]
public void Setup() {
// set up Mediator mock
_mediator = new Mock<IMediator>(MockBehavior.Strict);
// inject mock as dependency
_sut = new FeatureService(_mediator.Object);
}
[TestCleanup]
public void Teardown() {
// ensure we have called or expected all calls to Mediator
_mediator.VerifyAll();
}
[TestMethod]
public void ComplexBusinessLogic_Does_What_I_Expect() {
var dbObjects = new List<DbObject>() {
// set up any test objects
new DbObject() { }
};
// arrange
// setup Mediator to return our fake objects when it receives a message to perform our query
// in practice, I find it better to create an extension method that encapsulates this setup here
_mediator.Setup(x => x.Send(It.IsAny<GetRelevantDbObjectsQuery>(), default(CancellationToken)).ReturnsAsync(dbObjects.ToArray()).Callback(
(GetRelevantDbObjectsQuery message, CancellationToken token) => {
// using Moq Callback functionality, you can make assertions
// on expected request being passed in
Assert.IsNotNull(message);
});
// act
_sut.ComplexBusinessLogic();
// assertions
}
}
You can see all we need is a single setup and we don't even need to configure anything extra--it's a very simple unit test. Let's be clear: This is totally possible to do without something like Mediatr (you would simply implement an interface and mock it for tests, e.g. IGetRelevantDbObjectsQuery), but in practice for a large codebase with many features and queries/commands, I love the encapsulation and innate DI support Mediatr offers.
If you're wondering how I organize these classes, it's pretty simple:
- MyProject
- Features
- MyFeature
- Queries
- Commands
- Services
- DependencyConfig.cs (Ninject feature modules)
Organizing by feature slices is beside the point, but this keeps all relevant/dependent code together and easily discoverable. Most importantly, I separate the Queries vs. Commands--following the Command/Query Separation principle.
This meets all my criteria: it's low-ceremony, it's easy to understand, and there are extra hidden benefits. For example, how do you handle saving changes? Now you can simplify your Db Context by using a role interface (IUnitOfWork.SaveChangesAsync()) and mock calls to the single role interface or you could encapsulate committing/rolling back inside your RequestHandlers--however you prefer to do it is up to you, as long as it's maintainable. For example, I was tempted to create a single generic request/handler where you'd just pass an EF object and it would save/update/remove it--but you have to ask what your intention is and remember that if you wanted to swap out the handler with another storage provider/implementation, you should probably create explicit commands/queries that represent what you intend to do. More often than not, a single service or feature will need something specific--don't create generic stuff before you have a need for it.
There are of course caveats to this pattern--you can go too far with a simple pub/sub mechanism. I've limited my implementation to only abstracting EF-related code, but adventurous developers could start using MediatR to go overboard and message-ize everything--something good code review practices and peer reviews should catch. That's a process issue, not an issue with MediatR, so just be cognizant of how you're using this pattern.
You wanted a concrete example of how people are unit testing/mocking EF and this is an approach that's working successfully for us on our project--and the team is super happy with how easy it is to adopt. I hope this helps! As with all things in programming, there are multiple approaches and it all depends on what you want to achieve. I value simplicity, ease of use, maintainability, and discoverability--and this solution meets all those demands.
