Django：删除对象中的m2m关系

Question

I just started playing with Django, I love it! I am still working my way around with the Django ORM though...

At the moment I have a model Shift with a m2m relationship with users:

class Shift(models.Model):
    users = models.ManyToManyField(User) 

I want to define a view that checks if a M2M relationship exists, if it does, it removes this relationship. This is where I am stuck: I am able to lookup if a relationship exists, but I am not able to remove it. What is wrong with my code?

def remove_if_exists(request, shift_id, username):
    shift = get_object_or_404(Shift, pk=shift_id)
    if shift.users.filter(username=username).exists()
        shift.users.remove(username)

Answer 1

The trouble with your code is that the relationship is not with a username, but with a User object. So your call to remove should have a User object as its argument. You need to actually get the relevant user from the db first, then call remove with that object.

However, there is a shortcut: remove does not raise an error if the object is not in the related set, so you can skip the exists call. That gives just:

user = User.objects.get(username=username)
shift = get_object_or_404(Shift, pk=shift_id)
shift.users.remove(user)

Answer 2

我自己的一些愚蠢的语法错误应该是：

 shift.users.remove(User.objects.get(username=username))