Question

On my website any user have only one group. And any user can change his group.
So it’s made by

user.groups.clear() 

and

user.groups.add(new_group)

But it’s not efficient, because there is a two SQL query: DELETE, INSERT.
How can I change group by just UPDATE query?

Answer 1

User and Group are related to each other using a ManyToManyField. That means an intersection table exists relating both entities, and if you don’t specify a model to map to it (using the through attribute) Django creates one for you. Looking at the sources for django.contrib.auth.models I see that’s the case.

Fortunatly, you can access that intermediary model using the through attribute of the manager (in this case, User.groups.through). Then you can use it just like any regular model. Example:

>>> alice = User.objects.create_user('alice', '[e-mail]', 'alicepw')
>>> employee = Group.objects.create(name='employee')
>>> manager = Group.objects.create(name='manager')

>>> alice.groups.add(employee)
>>> alice.groups.all()
[<Group: employee>]

>>> alice_group = User.groups.through.objects.get(user=alice)
>>> alice_group
<User_groups: User_groups object>
>>> alice_group.group = manager
>>> alice_group.save()

>>> alice.groups.all()
[<Group: manager>]
>>>

(newlines added for readability)