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djangodjango-orm

Annotating without using Exists or SubQuery

I have a client who is using Django 1.8. While they will be moved to the latest version, we need to run some queries before their migration, but obviously we can't use Exists or OuterRef.

In our case we want to annotate a queryset. eg

recordset = Question.objects.annotate( has_answers=Exists(Answers.objects.filter(question=OuterRef('pk'))) )

Is there a workaround to do the equivalent of the above annotation. What did people use in 'the olden days'?

Solution

  • The following should work in 1.8, annotate each question with the count of answers and then use a conditional expression to convert that to a boolean

    from django.db.models import Count, Case, When, BooleanField

Question.objects.annotate(
    num_answers=Count('answer')
).annotate(
    has_answers=Case(
        When(num_answers__gt=0, then=True),
        default=False,
        output_field=BooleanField()
    )
)