var dep = _departmentRepository.Departments.FirstOrDefault(d => d.DepartmentId == Id); user.Department = dep; var result = await userManager.CreateAsync(user, model.Password); 虽然从主表Departments中找到了实例并赋值给了user的Department属性，但是CreateAsync的时候仍然要在主表Departments中添加此Department实例，怎么解决？急！