Mocking EF with async and Include

I'm trying to mock Entity Framework. And my method which include Async and working with 2 tables of EF. my method (MyClass.Create):

    using (var _db = new ApplicationDbContext())
                    {
                        var my = new Application(title, "", creatorId, documentId, deadLine);
                        var document = await _db.Documents.FindAsync(my.DocumentId);
//some stuffs
//....
                        _db.My.Add(my);
                        await _db.SaveChangesAsync();
    }

test:

        private ApplicationDbContext context;
        private DbSet<My> my;
        private DbSet<Document> document;
        private Document mDocument;
        [SetUp]
        public void Initialize()
        {
            // Instantiate mocks
            context = MockRepository.GenerateMock<ApplicationDbContext>();
            my = MockRepository.GenerateMock<DbSet<My>>();
            document = MockRepository.GenerateMock<DbSet<Document>>();
            mDocument = new Document(Guid.NewGuid().ToString(), "Про тест", "123456", Guid.NewGuid().ToString(), "12345", DateTime.Now, Guid.NewGuid().ToString(), Guid.NewGuid().ToString());
            // Setup unit of work to return mocked repository
            context.Stub(uow => uow.My).Return(my);
            context.Stub(uow => uow.Documents).Return(document);
        }
        [Test]
        public async Task Create_Consideration()
        {
            // Arrange
            document.Stub(doc => doc.FindAsync(Arg<int>.Is.Anything)).Return(Task.FromResult(mDocument));

            var r = await context.Documents.FindAsync(mDocument.Id);
            my.Expect(svc => svc.Add(Arg<My>.Is.Anything));
            context.Expect(svc => svc.SaveChanges());

            // Act
            await MyClass.Create("Test", mDocument.CreatorId, mDocument.Id);

            //Assert
            my.VerifyAllExpectations();
            context.VerifyAllExpectations();
        }
    }

Error what i get its: var document = await _db.Documents.FindAsync(my.DocumentId); document always null; But this, inside test var r = await context.Documents.FindAsync(mDocument.Id) ok;