django .filter（或任何其他管理器函数）来检查ManyToMany字段中是否存在项目

class Barrack(models.Model):
    """
    ... many other fields ...
    """

    def how_many_helmets_for_this_user(self, user):
        helmets = self.helmet_set.filter(possessors__contains=user) # how do we do this?
        return helmets.count()

class Helmet(models.Model):
    barrack = models.ForeignKey(Barrack)
    possessors = models.ManyToManyField(User)
class Barrack(models.Model):
    """
    ... ma