How to setup Mocked context using Moq?

Question

I have implemented a Repository Pattern using an onion Architecture and Entity Framework for Data Access, and now I want to test it using Moq. I just asked a question on SO and with the answer I am now more confused (the answer was good but I have very poor comprehension of how to mock even after reading doc). What I want to do is test Repository method Get(long id). My repository constructor takes a DbContext as parameter(called PrincipalServerContext, so I was suggested to mock the context in order to test my Repository. Let's say this is my Repository:

public class PrincipalServerContext : DbContext
{
    public DbSet<Web_Documents> WebDoc { get; set; }
    public PrincipalServerContext()
        : base("name=PrincipalServerDB")
    {
        Database.SetInitializer(new Initializer());
    }
 }

Now one of my POCOs Web_Documents (EF entity):

public class Web_Documents
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public long IDDocument { get; set; }

    [Required]
    [MaxLength(255)]
    public string NomDocument { get; set; }

    [Required]
    public long IDCategorie { get; set; }

    [ForeignKey("IDCategorie")]
    public Web_Categories cat { get; set; }
    [Required]
    [MaxLength(255)]
    public string Lien { get; set; }

    [MaxLength(50)]
    public string Type { get; set; }

    public virtual ICollection<Web_Profils> Profils { get; set; } 
}

And finally my Repository method (knowing that repository is generic, I use POCOs as Generic Types):

public T Get(long id)
{
    ObjectContext objContext = ((IObjectContextAdapter)context).ObjectContext;
    ObjectSet<T> set = objContext.CreateObjectSet<T>();
    IEnumerable<string> keyNames = set.EntitySet.ElementType
                                                .KeyMembers
                                                .Select(k => k.Name);
    if (keyNames.Count() > 1)
        return null;
    else
    {
        string idName = keyNames.ElementAt(0); // For Document would be IDDocument
        var parameter = Expression.Parameter(typeof(T));
        var property = Expression.Property(parameter, idName);
        var idValue = Expression.Constant(id, id.GetType());
        var equal = Expression.Equal(property, idValue);
        var predicate = Expression.Lambda<Func<T, bool>>(equal, parameter);
        return entities.SingleOrDefault(predicate);
        //Returns the corresponding entity to 'id' and 'T'
    }
}

This builds an expression with appropriate ID names because every table has a different ID Name (company's policy).

From what I was told here Should this case of Assert.AreSame return true? I understand that I have to build a return type for the Mock object, but my context class is so thin I don't have any methods or anything, just a DbSet. So I tried this as a test but it probably makes no sense since it failed (I'm just really lost and don't understand it):

Mock<PrincipalServerContext> moqContext;
public void IdExists(){
    moqContext = new Mock<PrincipalServerContext>();
    var set = new Mock<DbSet<Web_Documents>>();
    moqContext.Setup(c => c.Set<Web_Documents>()).Returns(set.Object);
    repoDoc = new Repository<Web_Documents>(moqContext.Object);
    var testDoc = repoDoc.Get(1L);
    Assert.AreEqual(testDoc.NomDocument, "Ajouter une catégorie");
}

Say I would want to make a simple test to find if the ID searched corresponds to my DB entry, how should I set the moqContext object I am trying to define? In examples I see they usually have methods for their mocked objects, but here none, so I found this Mocking DbContext for TDD Repository which made me try this test.

Thanks for your help!

Answer 1

Here's an example of using the in-memory database.

First you create an request an instance of the mock unit of work.

[TestMethod]
public async Task ExampleTest() {
    //arrange
    Mock<IUnitOfWork> mockUow = MockUowFactory.Get(nameof(ExampleTest));

    //act
    using (var app = YOURAPP(mockUow.Object)){
         app.METHODUNDERTEST();
    }

    //assert
    ...
}

Then you build up the mock unit of work. According to what I've read the separate context is needed (one for seeding, one for testing). The MockEntityFactory just returns an array of dummy data which is used to populate the dbsets in our InMemoryDatabase.

    public class MockUowFactory {

    public static Mock<IUnitOfWork> Get(string dbName) {

        DbContextOptions<YOUR CONTEXT> options = new DbContextOptionsBuilder<YOUR CONTEXT>()
            .UseInMemoryDatabase(databaseName: dbName)
            .Options;


        using (var seedContext = new YOURCONTEXT(options)) {

            seedContext.YOURENTITY.AddRange(MockEntityFactory.YOURENTITY);

            seedContext.SaveChanges();
        }

        var context = new YOURCONTEXT(options);
        var mockUow = new Mock<IUnitOfWork>();
        mockUow.Setup(m => m.Context).Returns(context);
        mockUow.Setup(m => m.Save()).Returns(() => context.SaveChanges().ToString());

        return mockUow;
    }
}

I then pass this unit of work through the necessary layers and don't need to do anything special to test my production code.