QuerySet.union() crashes with IndexError when getting cached results.

[Raphaël Stefanini]

I tried 4.2a1 today on my project and I noticed pagination was broken on one view.

After little investigation I manage to reproduce an error:

Event is a model with set a Foreignkey

>> events = Event.objects.all()
>> events_without_set = events.filter(set__isnull=True)
>> one_event_of_each_set = (
        events.filter(set__isnull=False)
               .order_by("set_id")
               .distinct("set_id")
)
>> my_list = events_without_set.union(one_event_of_each_set)
>> my_list.count()
212
>> my_list[2]
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/venv/lib/python3.11/site-packages/django/db/models/query.py", line 450, in __getitem__
    return qs._result_cache[0]
           ~~~~~~~~~~~~~~~~^^^
IndexError: list index out of range

With django 4.1.5 my_list[2] returns an event.

[Mariusz Felisiak]

Thanks for testing the alpha release!

Regression in 3d734c09ff0138441dfe0a59010435871d17950f.

[Simon Charette]

It seems that the issue is likely due the ​if sliced branch which was hoping to port the optimization that made union().exists() have all the involved subqueries augmented with LIMIT 1 if supported by the backend.

I believe that this optimization can likely be removed since backends are likely smart enough to perform this optimization themselves when EXISTS is used.

[Francesco Panico]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In cc8aa6bf:

Fixed #34267 -- Fixed sliced QuerySet.union() crash.

Regression in 3d734c09ff0138441dfe0a59010435871d17950f.

Thanks Raphaël Stefanini for the report.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 84927e1:

[4.2.x] Fixed #34267 -- Fixed sliced QuerySet.union() crash.

Regression in 3d734c09ff0138441dfe0a59010435871d17950f.

Thanks Raphaël Stefanini for the report.

Backport of cc8aa6bf9c127a493e6dd005012b9e6397b3f319 from main