Include known related fields for subclassed "QuerySet.only"

[lvo@…]

I have a model like these:

class PNRSegment(models.Model):
    pnr = models.ForeignKey('booking.PassengerNameRecord', related_name="segments")
    arrival_datetime = models.DateTimeField()

    objects = PNRSegmentManager()

Also i wrote my own Manager with own QuerySet:

class PNRSegmentManager(models.Manager):
    use_for_related_fields = True

    def get_query_set(self):
        return PNRSegmentQuerySet(self.model, using=self._db)

    def latest_datetime(self):
        return self.get_query_set().latest_datetime()


class PNRSegmentQuerySet(QuerySet):
    def latest_datetime(self):
        try:
            return self.order_by('-arrival_datetime')[0].arrival_datetime
        except IndexError:
            return None

I noticed that QuerySet.only has strange behavior:

# When launching pdb into PNRSegmentQuerySet.latest_datetime:
(Pdb) pp self
[<PNRSegment: PNRSegment object>, <PNRSegment: PNRSegment object>]
(Pdb) pp self.count()
2
(Pdb) pp self[0]
<PNRSegment: PNRSegment object>
(Pdb) pp self.only('arrival_datetime').count()
2
(Pdb) pp self.only('arrival_datetime')[0]
*** IndexError: IndexError(("PNRSegment matching query does not exist. Lookup parameters were {'pk': None}",),)

After tracing of QuerySet.clone i found my error: when subclassing QuerySet for related manager, each QuerySet.only call should contain related field:

(Pdb) pp self.only('arrival_datetime', 'pnr')[0]
<PNRSegment_Deferred_airline_code_class_code_ ... cutted ...>

I think that this is not obvious behaviour. QuerySet.only should authomatically include all fields from QuerySet._known_related_objects.

[sduveen]

only() is about the fields returned. It's specifically used to limit fields and you use it so that it won't include additional ones. If you want defaults, then don't use only()

[sduveen]

seeing if we can make a useful test

[sduveen]

seems to be working in 1.6 and master.

This test shows we can do a query that has a manager and foreignkey object.
​https://github.com/schuyler1d/django/commit/047c131bc96a844d2769f3a4c5612119330413a1