for the_drow in #django IRC. I can't remember whether any() will bail at the first `True`, which would presumably be the quickest exit.

prefetch_abuse.py

class MyThing(Model):
    def has_value_x_y(self):
        assert hasattr(self, '_prefetched_objects_cache'), \
            "needs prefetch_related()"
        # `my_thing__set` becomes `my_thing` in the prefetch cache ...
        assert 'RELATION' in self._prefetched_objects_cache, \
            "needs prefetch_related('RELATION_set')"
        count = (True for x in self.RELATION_set.all())
        return any(count)
        
 # will be an AssertionError to remind you not to cross relations without doing the right thing
MyThing.objects.all()[0].has_value_x_y()

# this will be fine, but will do all the work to get all the results. Useful if mostly you want
# to act on the related data afterwards anyway.
MyThing.objects.prefetch_related('RELATION_set')[0].has_value_x_y()