public async Task <IActionResult> Add(AssignmentDetailsDto assignmentDto)
{
Subject subject = await _db.Subjects.FindAsync(assignmentDto.SubjectId);
if (subject.TeacherId != _userService.UserId)
{
return(Forbid());
}
Assignment assignment = await _assignmentService.AddAsync(assignmentDto, subject);
await _db.SaveChangesAsync();
return(CreatedAtAction(nameof(GetById), new { assignment.Id }, _mapper.Map <AssignmentDetailsDto>(assignment)));
}