数据表中有2个字段都指向一个外键

class Student(models.Model):
  old_school = model.ForeignField(School)
  school = model.ForeignField(School)

我想找出所有old_school == school的行，请问这个怎么处理？
我试过了用：

Student.objects.all().exclude(old_school = school)
Student.objects.all().exclude(old_school__id = school__id)
Student.objects.all().exclude(old_school = self.school)

都不行。