Allow UserCreationForm and UserChangeForm to work with custom user models

[Rômulo Collopy]

class UserChangeForm(forms.ModelForm):
    # ...
    class Meta:
        model = User

and

class UserCreationForm(forms.ModelForm):
    class Meta:
        model = User

could use UserModel instead of User. It is already defined in ​https://github.com/django/django/blob/01c6a3e227b645e8dea97e9befecd23d1d3b8581/django/contrib/auth/forms.py#L20 and is used in other forms of the same package.

This would allow Django Registration and other packages that use these forms to work out of the box. And it would allow using them with no need to rewitte as specified at ​https://docs.djangoproject.com/en/1.11/topics/auth/customizing/#custom-users-and-the-built-in-auth-forms

[Rômulo Collopy]

patch

[Tim Graham]

This is a continuation of #19353.

[shangdahao]

Since This ticket is related to ticket 28757, I made the changes for this ticket and ticket 28757 in the same commit.

​PR

[Tim Graham <timograham@…>]

In 3333d93:

Fixed #28757 -- Allowed using contrib.auth forms without installing contrib.auth.

Also fixed #28608 -- Allowed UserCreationForm and UserChangeForm to
work with custom user models.

Thanks Sagar Chalise and Rômulo Collopy for reports, and Tim Graham
and Tim Martin for reviews.

[Tim Graham]

In f3fa86a89b3b85242f49b2b9acf58b5ea35acc1f:

Fixed #29449 -- Reverted "Fixed #28757 -- Allowed using contrib.auth forms without installing contrib.auth."

This reverts commit 3333d935d2914cd80cf31f4803821ad5c0e2a51d due to a crash if USERNAME_FIELD isn't a CharField.

[Lemuel Formacil]

Another non-standard behavior of the UserCreationForm when used with a custom user model is if the model has a ManyToManyField the form will not save the value of the ManyToManyField. From the ​documentation:

If your model has a many-to-many relation and you specify commit=False when you save a form, Django cannot immediately save the form data for the many-to-many relation. This is because it isn’t possible to save many-to-many data for an instance until the instance exists in the database.

To work around this problem, every time you save a form using commit=False, Django adds a save_m2m() method to your ModelForm subclass. After you’ve manually saved the instance produced by the form, you can invoke save_m2m() to save the many-to-many form data.

However, the UserCreationForm.save() method initially calls the form's save method with commit=False but then doesn't call the save_m2m method after committing the instance if commit=True. The save() method should be something like this so the form would behave as expected with custom user models with a many-to-many relation:

def save(self, commit=True):
    user = super().save(commit=False)
    user.set_password(self.cleaned_data["password1"])
    if commit:
        user.save()
        self.save_m2m()  # added this call to work as expected with models with `ManyToManyField`s
    return user

[Sarah Boyce]

#35484 is a duplicate, this generally tracks the work to make the UserAdmin work for custom user models 👍