django – 如何删除仅m2m关系？

class Province(models.Model):
    user = models.ManyToManyField(User,blank=True)
    name = models.CharField(max_length=30,unique=True)

class City(models.Model):
    name = models.CharField(max_length=100,unique=True)
    slug = models.SlugField(max_length=100,editable=False,unique=False)
    ownership = models.ManyToManyField(User,through='UserCity')


class UserCity(models.Model):
    user = models.ForeignKey(User)
    province = models.ForeignKey(Province)
    city = models.ForeignKey(City)


class District(models.Model):
    name = models.CharField(max_length=100,unique=True,editable=False)
    ownership = models.ManyToManyField(User,through='UserDistrict')

class UserDistrict(models.Model):
    user = models.ForeignKey(User)
    province = models.ForeignKey(Province)
    city = models.ForeignKey(City)
    district = models.ForeignKey(District)

当我知道user_id和province_id时,如何删除关系？如果我用户delete()方法它也删除省,我想避免它.我找不到任何地方如何删除m2m字段中的1个特定关系.