id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
20886	auth.forms.AuthenticationForm incompatible with auth.backends.ModelBackend when using custom user model	Paul Garner	nobody	"If you have a custom user model which does not have a field called `username` you are supposed to be able to use a different field (eg `email`) and specify `USERNAME_FIELD = 'email'` on your custom model.

I'm trying to use the built-in login view. It looks fine when I bring up the page, it has recognised my `USERNAME_FIELD` and displays 'Email address' as the label.

However I can't login. I have looked into why - it's because there is this code in `auth.forms.AuthenticationForm`

{{{
    def clean(self):
        username = self.cleaned_data.get('username')
        password = self.cleaned_data.get('password')

        if username and password:
            self.user_cache = authenticate(username=username,
                                           password=password)
}}}

...where `username` here is the name of the field on the form and is not wrong. The problem is when it calls `authenticate(username=username...`

The `authenticate` method of `ModelBackend` looks like:

{{{
    def authenticate(self, username=None, password=None, **kwargs):
        UserModel = get_user_model()
        if username is None:
            username = kwargs.get(UserModel.USERNAME_FIELD)
}}}

which tries to integrate our custom `USERNAME_FIELD`... but we can see we're doing it wrong back in `AuthenticationForm`.

"	Bug	closed	contrib.auth	1.5	Normal	invalid			Unreviewed	0	0	0	0	1	0