Make an abstract verstion of django's UserSocialAuth's model so it can be extended

[troygrosfield]

The request here is to make an abstract version of django's UserSocialAuth model so it can be extended:

• https://github.com/omab/python-social-auth/blob/master/social/apps/django_app/default/models.py

The current model is great, but when I encounter things like facebook access tokens that expire, I need to store the datetime that the tokens expire so I can query them and extend them. I don't want to store this in the extra_data field because it's stored as a text field and is not queryable. This would allow me to leverage the existing model and just add the extra fields that are needed.

The change for this is super trivial:

class AbstractUserSocialAuth(models.Model, DjangoUserMixin):
    user = models.ForeignKey(USER_MODEL, related_name='social_auth')
    provider = models.CharField(max_length=32)
    uid = models.CharField(max_length=UID_LENGTH)
    extra_data = JSONField()

    def __str__(self):
        return str(self.user)

    class Meta:
        abstract = True

    ...
    The rest of the current UserSocialAuth model
    ...

class UserSocialAuth(AbstractUserSocialAuth):

    class Meta:
        unique_together = ('provider', 'uid')
        db_table = 'social_auth_usersocialauth'

The change is passive for others and lets people leverage the existing work done for the UserSocialAuth model.

Then people can easily create their own versions of the UserSocialAuth model:

class MyUserSocialAuth(AbstractUserSocialAuth):
    token_expires = models.DateTimeField(...)
    last_modified_dttm = models.DateTimeField(...)

    class Meta:
        unique_together = ('provider', 'uid')

@omab, thoughts?