public async Task AddExistCourseTest()
        {
            var options = new DbContextOptionsBuilder <ApplicationDbContext>()
                          .UseInMemoryDatabase(Guid.NewGuid().ToString());
            var repository     = new EfDeletableEntityRepository <Course>(new ApplicationDbContext(options.Options));
            var userRepository =
                new EfDeletableEntityRepository <ApplicationUser>(new ApplicationDbContext(options.Options));
            var user = new ApplicationUser
            {
                FullName = "Test",
            };

            var course = new Course
            {
                Title = "test",
            };
            await userRepository.AddAsync(user);

            await userRepository.SaveChangesAsync();

            await repository.AddAsync(course);

            await repository.SaveChangesAsync();

            var coursesService = new CoursesService(repository, userRepository);

            coursesService.AddCourseAsync(course.Id, user.Id, 1, 1);
            coursesService.AddCourseAsync(course.Id, user.Id, 1, 1);
            Assert.Single(user.Courses);
        }