[Django]-Additional conditions on join in django

In Django v2.0 use FilteredRelation

Post.objects.annotate(
    t=FilteredRelation(
        'watchlist', condition=Q(watchlist__user_id=1)
).filter(t__field__in=...)

Short answer: in certain conditions – yes.

When construction LEFT JOINs with GenericForeignKey, Django calls
GenericRelation.get_extra_restriction which adds extra condition to ON clause with “content_type_id” restriction.

For “ForeignKey” this method is also called by returns None.

You may use this place to put extra restrictions into ON clause, if you manage to organize your code to get proper restriction params at certain time.

class UserForeignKey(models.ForeignKey):

    def get_extra_restriction(self, where_class, alias, related_alias):
        field = self.model._meta.get_field('user')
        cond = where_class()
        # Here is a hack to get custom condition parameters
        value = SomeContextManager.get_needed_value()

        lookup = field.get_lookup('exact')(field.get_col(related_alias), value)
        cond.add(lookup, 'AND')
        return cond

class WatchList(models.Model):

    user = UserForeignKey(User)

Post.objects.annotate(Count('watchinglist')).filter(pk=123).extra(where=['"watchlist"."user_id" = 1'])

Happy Coding.