Empty Querset Using Empty Queryset in ExpressionWrapper

[Sven R. Kunze]

We use the ExpressionWrapper + annotate to annotate specific boolean values to all queried instances like this (minimal working example):

editable_ones = ..... # another queryset
result = MyModel.objects.annotate(editable=ExpressionWrapper(Q(pk__in=editable_ones), output_field=BooleanField())

As soon as editable_ones is empty, result is also empty. Reading the docs, I guess this is not desired behavior (​https://docs.djangoproject.com/en/1.9/ref/models/querysets/#annotate).

[Sven R. Kunze]

I did some further analysis:

MyModel.objects.annotate(editable=ExpressionWrapper(Q(pk__in=MyModel.objects.none()), output_field=BooleanField())

MyModel.objects.annotate(editable=ExpressionWrapper(Q(pk__in=MyModel.objects.filter(pk=-1)), output_field=BooleanField())

Both results differ.

MyModel.objects.none() forces the outer query to return an empty QuerySet.

[Simon Charette]

Related to or duplicate of both #26430 and #26433.

[Simon Charette]

​PR

[Josh Smeaton]

FYI ExpressionWrapper was never meant to be used to wrap up Q() objects. Q() are not really expressions themselves, they just implement some of the same API internally.

You're much better off using Case Expressions ​https://docs.djangoproject.com/en/1.9/ref/models/conditional-expressions/ for this use case:

MyModel.objects.annotate(editable=Case(When(pk__in=[editable], then=1), default=0, output_field=BooleanField()) )

[Tim Graham <timograham@…>]

In c002a0d3:

Fixed #26517 -- Fixed ExpressionWrapper with empty queryset.