Django模型字段内容怎么约束

[景韦]

一张教师表，一张班级表

class Teacher(models.Model):
    name = models.CharField(max_length=10)
    subject = models.CharField(max_length=10)

class Classname(models.Model):
    grade = models.CharField(max_length=10)
    class_no = models.PositiveSmallIntegerField()
    teacher_chinese = models.ForeignKey(Teacher,
                                        on_delete=models.PROTECT,
                                        related_name='teacher_c')
    teacher_math = models.ForeignKey(Teacher,
                                     on_delete=models.PROTECT,
                                     related_name='teacher_m')
    teacher_english = models.ForeignKey(Teacher,
                                        on_delete=models.PROTECT,
                                        related_name='teacher_e')

    class Meta:
        unique_together = ('grade', 'class_no')

班级的teacher_chinese只能是teacher表中subject是语文的老师
请问如何实现？

[ruancan]

在插入数据的时候加判断吧