1.2 breaks Prefetch compatibility for Django 1.10-

[antwan]

When using custom Prefetch with polymorphic model, this is now failing.

Prefetch('field', queryset=Fieldmodel.objects.filter(somefilter=somevalue))

ValueError('Prefetch querysets cannot use values().')

Django 1.11 works here because it's checking the subclasses of queryset iterator, but 1.10 is broken.
Maybe it's worth fixing this, or document that polymorphic 1.2 requires Django 1.11+.

[vdboor]

Do you have an code example how to trigger this bug?

[antwan]

class OtherModel(PolymorphicModel):
    myfield = models.IntegerField()

class MyModel(models.Model):
    myforeignfield = models.ForeignKey(OtherModel)

MyModel.objects.prefetch_related(
    'myforeignfield',
    queryset=OtherModel.objects.filter(myfield=value),
    to_attr='myforeignfield_with_value'
)

[rmeyers4]

I have only seen this when trying to prefetch objects with custom managers that are related to a PolymorphicModel via a ManyToManyField.

class ModelA(PolymorphicModel):
      field = models.ManyToManyField(ModelB)

class ModelB(models.Model):
      somefield = models.IntegerField()

ModelA.objects.prefetch_related('modelb', ModelB.objects.filter(somefield=somevalue))

.../django/db/models/query.pyc in __init__(self, lookup, queryset, to_attr)
   1334         self.prefetch_to = lookup
   1335         if queryset is not None and queryset._iterable_class is not ModelIterable:
-> 1336             raise ValueError('Prefetch querysets cannot use values().')
   1337         if to_attr:
   1338             self.prefetch_to = LOOKUP_SEP.join(lookup.split(LOOKUP_SEP)[:-1] + [to_attr])

ValueError: Prefetch querysets cannot use values().

[rmeyers4]

After a little more investigation, I am only seeing this issue when trying to prefetch objects with custom managers that override _iterable_class. Not sure if this fits with the experience of others.

[vdboor]

Hmm, polymorphic also re-overrides the _iterable_class in PolymorphicQuerySet.__init__(). You may want to redefine it, and inherit our PolymorphicModelIterable, or combine both classes.

Does that solve your issue?

[vdboor]

I also think Django shouldn't use queryset._iterable_class is not ModelIterable but instead use isinstance()! That is a bug in Django too.

[brunobarretofreitas]

A workaround solution is setting the _iterable_class of the queryset to ModelIterable

from django.db.models.query import ModelIterable

prefetch_queryset= PrefetchModel.object.all()
prefetch_queryset._iterable_class = ModelIterable

PolymorphicModel.objects.prefetch_related('prefetch_related_field', queryset=prefetch_queryset)