len(queryset) and queryset.count() return different results

[Sander Kleijwegt]

According to the queryset.count() documentation "you should always use count() rather than loading all of the record into Python objects and calling len() on the result" (see ​here), which seems to imply that the results of both functions should always be identical. Here is a simple example that shows the contrary.

Models:

class Bar(models.Model):
        pass

class Foo(models.Model):
        fooint = models.IntegerField()
        foobar = models.ForeignKey(Bar, related_name='foos', on_delete=models.CASCADE)

Shell Output:

>>> import django
>>> django.__version__
'2.0.4'
>>> from foobar.models import Foo, Bar
>>> bar = Bar.objects.create()
>>> foo1 = Foo.objects.create(fooint=1, foobar=bar)
>>> foo2 = Foo.objects.create(fooint=2, foobar=bar)
>>> Bar.objects.all().order_by('foos__fooint').count()
1
>>> len(Bar.objects.all().order_by('foos__fooint'))
2
>>> Bar.objects.all().order_by('foos__fooint')
<QuerySet [<Bar: Bar object (1)>, <Bar: Bar object (1)>]>