如何使用Lambda表达式进行单元测试实体框架/存储库模式

Question

We are trying to figure out what the point of unit testing is for basic cases as in the code below. Does making a unit test for this beneficial ? We are not trying to test the entity framework. We just want to make sure the lambda expression does what it should... Our idea is that we will use DI to pass in SOMETHING that is IQueryable.. In practice it will be EF but for unit tests and will be POCO objects/collections. Does this makes sense ? We are just getting started and want to master the concepts before we get beyond this basic code.

   public class CongressRepository
{
    CongressDb_DevEntities context = new CongressDb_DevEntities();

    CongressRepository(DbContext db)
    {
        context = (CongressDb_DevEntities) db;
    }

    public IQueryable<tMember> GetAllMembers
    {
        get { return context.tMembers; }
    }

    public IQueryable<tMember> GetVotingMembers
    {
        get { return context.tMembers.Where(x => x.age > 18); }
    }
}

Answer 1

EF uses LINQ to Entities, but when mocking EF you'd be switching to LINQ to Objects. This could lead to false positives in terms of your unit tests as there are differences between LINQ to Entities and LINQ to Objects. Without integration tests, you'd only see these differences / errors in your production environment.

Answer 2

One of the reasons for unit testing code is ensuring accuracy...

In your example someone could not vote on their 18th birthday or any of the days between it and their 19th birthday. I'm guessing it should really be:

public IQueryable<tMember> GetVotingMembers
{
    get { return context.tMembers.Where(x => x.age >= 18); }
}

Unit tests which cover a range of reasonable successful and failing values help ensure that your code does what you meant it to :-)

---

but what we are trying to find a why to ask is how the unit tests work.

It can be a real pain! EF6 has just been announced and makes it easier to mock the DbContext but right now it takes a lot of setup...

The important things is that there are differences between Linq-to-Etities and Linq-to-Objects so you do need both unit testing and integration testing.

There are lots of blogs and SO questions related though... use FakeDbSet as a starting point for some google-fu.

Answer 3

For unit testing an object that uses DbContext, you will have to inject it as an interface into your repository object. Follow these steps:

1.Create an interface that contains all DbContext properties, and derive your DbContext from it:

 public interface ICongressDbContext {
    IDbSet<Member> Members { get; set; }
    // repeat for all dbsets
 }

2.Give this interface to your repository in the constructor. This is your dependency injection:

 private ICongressDbContext context;
 public CongressRepository(ICongressDbContext context){
     this.context = context;
 }

You will want to pass an instance of the context in using a dependency injection framework like Autofac, or you can create an instance where you instantiate your object.

3.Create a generic implementation of IDbSet: https://gist.github.com/LukeWinikates/1309447 . Let's call this FakeDbContext.

4.Now that you have a mockable interface, and a generic DbSet implementation, you are ready to create your mock DbContext and your mock IDbSets. Using Moq, this looks like:

        var dataContext = new Mock<ICongressDbContext>();
        var members = new List<Member> { // create your test data here };
        // set up the mocked context to use your fake db set with test data
        dataContext.SetupProperty(c => c.Members, new FakeDbContext<Member>(members));

        var repository = new CongressRepository(dataContext.Object);

        // start writing your test methods here

This will test the object that uses the DbContext, not the DbContext itself.
You will want to write integration tests that use the real DbContext in addition to the unit tests. There are many methods supported on IQueryable that are not supported when accessing the database. You also want to make sure your data model is mapped correctly to the database.

I hope this helps you to get started.