unexpected behavior when using gettattr for related object

[elonzh]

Assuming we have a model like that:

class UserProfile(models.Model):
    user = models.OneToOneField(User, related_name="profile", on_delete=models.CASCADE)
    ...

An user may not have an UserProfile instance so we will use such logic:

profile = getattr(request.user, "profile", UserProfile(user=request.user))

But django will always return the default value no matter request.user.profile exists or not.

Here is the poc:

In [1]: u = User.objects.get(username='s')

In [2]: u.profile
Out[2]: <UserProfile: UserProfile object (5)>

In [3]: getattr(u, 'profile', UserProfile())
Out[3]: <UserProfile: UserProfile object (5)>

In [4]: getattr(u, 'profile', UserProfile(user=u))
Out[4]: <UserProfile: UserProfile object (None)>

the problem is UserProfile(user=u) will update the u.profile when we init UserProfile.

I know this issue is caused by design, maybe we should update the document for such case.

[Mariusz Felisiak]

The problem is UserProfile(user=u) will update the User.profile when we init UserProfile.

As far as I'm aware this is an expected behavior to keep relationships in sync. I don't think there is anything that we could improve in docs, there is also ​an example which shows this behavior:

>>> p1 = Place(name='Demon Dogs', address='944 W. Fullerton')
>>> p1.save()
>>> p2 = Place(name='Ace Hardware', address='1013 N. Ashland')
>>> p2.save()
>>> r = Restaurant(place=p1, serves_hot_dogs=True, serves_pizza=False)
>>> r.save()
>>> r.place
<Place: Demon Dogs the place>
>>> p1.restaurant
<Restaurant: Demon Dogs the restaurant>

You can use hasattr() or catch AttributeError.