Model FormSet & Prefetch ordered

[Adrien mille]

I happened to meet some unexpected behavior while mixing together the Model FormSet with a Queryset on which I had prefetch.

My setup is the following one ;

prefetch = Prefetch('childs', Child.objects.order_by('order')),
queryset = Parent.objects.prefetch_related(prefetch)
parent = queryset.get(pk=42)
formset = ChildFormSet(queryset=parent.childs.all())

Everything works without warning or error, however the formset does not respect the queryset I provide. I tried to understand a bit more about the reasons behind it and I finally see the folloxing lines in BaseModelFormSet.get_queryset() ;

# If the queryset isn't already ordered we need to add an
# artificial ordering here to make sure that all formsets
# constructed from this queryset have the same form order.
if not qs.ordered:
     qs = qs.order_by(self.model._meta.pk.name)

So I went to read my queryset object and I saw that the property ordered is to False while the results are correctly ordered (the property _prefetch_done is True on my queryset object). So my thinking is that the queryset resulting of parent.object.all() is my prefetch (as expected) but not ordered as my FormSet expects. So it does order it by the primary key for the model and breaks my prefetch.

I checked twice on the documentation and source code but I didn't see any warning about putting together prefetch & formset, especially on ordered queryset.