QuerySet.union() with queryset that raises EmptyResultSet results in an empty set

[Jon Dufresne]

Similar and followup to ticket #28293. Tested on Django 1.11.3.

As QuerySet.union() uses the SQL UNION operator, I would expect "SET1 UNION <EMPTY SET>" to result in SET1. If the empty set is the results of ​EmpytResultSet being raised, the .union() result is an empty set, not SET1.

Example test case:

>>> import django
>>> django.__version__
'1.11.3'
>>> from django.contrib.auth.models import User
>>> qs1 = User.objects.all() 
>>> qs1.count()
100
>>> qs2 = User.objects.filter(pk__in=[])
>>> qs2.count()
0
>>> list(qs1.union(qs2))
[]

[Mariusz Felisiak]

difference is also affected.

[Mariusz Felisiak]

I attached tests.

[Mariusz Felisiak]

​PR

[Simon Charette]

Patch LGTM, it's only missing release notes.

[Mariusz Felisiak <felisiak.mariusz@…>]

In ca74e563:

Fixed #28378 -- Fixed union() and difference() when combining with a queryset raising EmptyResultSet.

Thanks Jon Dufresne for the report. Thanks Tim Graham and Simon Charette
for the reviews.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 346b46a:

[1.11.x] Fixed #28378 -- Fixed union() and difference() when combining with a queryset raising EmptyResultSet.

Thanks Jon Dufresne for the report. Thanks Tim Graham and Simon Charette
for the reviews.

Backport of ca74e563500e291480f1976b58fcd34aac768dca from master