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); }