QuerySet.count() raises ValueError when QuerySet.union() is used

[kapil garg]

When we get objects from a model and do union then the resulted QuerySet.count() raises ValueError due to the presence of Non-Numeric first Field.

Eg.

class SomeModel(models.Model):
    name = models.CharField(max_length=10, default='example')

>>> from bug.models import SomeModel
>>> e1 = SomeModel(name='current')
>>> e1.save()
>>> e2 = SomeModel()
>>> e2.save()
>>> qs1 = SomeModel.objects.filter(name__startswith='ex').only('name')
>>> qs2 = SomeModel.objects.filter(name__startswith='cu').only('name')
>>> qs = qs1.union(qs2)
>>> print(qs.count())
>>> #raises ValueError : invalid literal for int() with base 10

If the first field contained in queryset is numeric then this error is not raised.

Eg. remove .only() method above on both querysets and then .count() with return correct result.

[kapil garg]

I found the reason. union() does not support count().

[Tim Graham]

I opened #27995 to raise a descriptive error on unsupported operations following QuerySet.union().