Mocking DBSet, EF Model First - unit-testing

Question

As said in the title, I follow Model First method. So my Model classes are Automatically generated. If I want mock the DBContext derived MyModelContainer which contain DBSets of entity classes. Read some where that in order to unit test, you need to change it to IDBSet. Whether its possible to do it especially in a class that gets auto generated when I do "Run Custom Tool" is one concern. But as of now I modified it.
But the real problem is: when I try to Stub MyModelContainer to return a mock generated from IDBSet. Rhino mock is firing an InvalidOperationException: "Invalid call, the last call has been used, or no call has been made(make sure that you are calling a virtual(C#)/Overridable(VB) method."
Here is my unit test code.
MyModelContainer dbMock = MockRepository.GenerateMock<MyModelContainer>();
IDBSet<Models.MyEntity> entityMock = MockRepository.GenerateMock<IDBSet<Models.MyEntity>>()
dbMock.Stub( x=>x.MyEntities ).Return( entityMock );
The last statement is triggering the exception. I tried using the fake implementation of IDBSet<> specified here, But no luck!
I use MVC 4, Rhino Mocks 3.6. Any help will be appreciated.
Update:
After some trials and research, I found a fix. I changed the code to:
MyModelContainer dbMock = MockRepository.GenerateMock<MyModelContainer>();
IDBSet<Models.MyEntity> entityMock = MockRepository.GenerateMock<IDBSet<Models.MyEntity>>()
//dbMock.Stub( x=>x.MyEntities ).Return( entityMock );
dbMock.MyEntities = entityMock;
Now the InvalidOperationException is gone.
The test fails only due to ExpectationViolationException which should be normal.
As for auto generated Model class, it is found out that editing the DbContext's T4 template (.tt extension) will do the trick. Thanks to Alan's Blog
But I want to know why the previous code didn't work. Anyone?

Answer 1

2 reasons are possible here:
MyEntites property of MyModelContainer is not virtual.
In that case Rhino Mock can't stub this property at all. Then dbMock.Stub(x=>x.MyEntities) will fail.
MyEntites property is virtual, but has both public getter and public setter.
Then notation dbMock.Stub(x=>x.MyEntities).Return(entityMock) is not allowed. You can see explanation e.g. here.
In both cases the right fix is exactly what you did: use dbMock.MyEntities = entityMock instead of dbMock.Stub(x=>x.MyEntities).Return(entityMock).

Answer 2

Here is an extension method for Substituting IDbSet (with NSubstitute) to return an IQueryable
public static DbSet<T> FakeDbSet<T>(this IQueryable<T> queryable) where T : class
{
DbSet<T> fakeDbSet = Substitute.For<DbSet<T>, IQueryable<T>>();
((IQueryable<T>)fakeDbSet).Provider.Returns(queryable.Provider);
((IQueryable<T>)fakeDbSet).Expression.Returns(queryable.Expression);
((IQueryable<T>)fakeDbSet).ElementType.Returns(queryable.ElementType);
((IQueryable<T>)fakeDbSet).GetEnumerator().Returns(queryable.GetEnumerator());
fakeDbSet.AsNoTracking().Returns(fakeDbSet);
return fakeDbSet;
}
Then you can now stub the DbContext like this:
var db = NSubstitute.Substitute.For<DataContext>();
var fakeResult = emptyCustomers.FakeDbSet();
db.Customers.Returns(fakeResult);

Answer 3

Here is an extension method for Stubing (with RhinoMocks) IDbSet to return an IQueryable
public static class RhinoExtensions
{
public static IDbSet<T> MockToDbSet<T>(this IQueryable<T> queryable) where T : class
{
IDbSet<T> mockDbSet = MockRepository.GenerateMock<IDbSet<T>>();
mockDbSet.Stub(m => m.Provider).Return(queryable.Provider);
mockDbSet.Stub(m => m.Expression).Return(queryable.Expression);
mockDbSet.Stub(m => m.ElementType).Return(queryable.ElementType);
mockDbSet.Stub(m => m.GetEnumerator()).Return(queryable.GetEnumerator());
return mockDbSet;
}
}
Then you can now stub the DbContext like this:
_db.Stub(p => p.Customers).Return(fakeCustomers.MockToDbSet());