Reactive joins for publications with `user_rel`

[tysonclugg]

(From https://gitter.im/commoncode/django-ddp?at=56551f41bcd12a10513a0be3)

Given the Assessment model:

class Assessment(models.Model):
    invited = models.ManyToManyField('auth.user')

Then the following collection will only send Assessments to each user in invited:

class Assessment(Collection):

    """Assessment DDP Collection."""

    model = assessment.Assessment
    user_rel = [
       'invited',
    ]

Django DDP will allow the object to be seen if the user is mentioned in ANY of the fields referenced in user_rel (it applies the OR operator).

Unfortunately I haven't figured out a way to efficiently follow joins in publications yet: we currently have no reactive joins.

Oh hey, I just figured out how to do efficient reactive user_rel publications!

Note to future self implementing reactive user_rel publications:

• Only re-process publications for the users mentioned in pre and post change.

eg: signals.m2m_changed(action='pre_add', instance=<User>) -> only re-process subscriptions for <User>.