Django queryset annotate field to be a list/queryset

Question

I'm trying to use django annotation to create queryset field which is a list of values of some related model attribute.

queryset = ...
qs = queryset.annotate(
    list_field=SomeAggregateFunction(
        Case(When(related_model__field="abc"), then="related_model__id")
    ),
    list_elements=Count(F('list_field'))
)

I was thinking about about concatenating all these id with some separator, but i don't know the appropriate functions. Another solution is to make list_field a queryset. I know this syntax is wrong. Thank you for any help.

Answer 1

If you are using postgresql and django >= 1.9, you could use postgres specific aggregating functions e.g. ArrayAgg:

Returns a list of values, including nulls, concatenated into an array.

In case, you need to concatenate these values using a delimiter, you could also use StringAgg.

Answer 2

I have done something like that:

qs = queryset \
    .annotate(
        field_a=ArrayAgg(Case(When(
            related_model__field="A",
            then="related_model__pk")
        )),
        field_b=ArrayAgg(Case(When(
            related_model__field="B",
            then="related_model__pk")
        )),
        field_c=ArrayAgg(Case(When(
            related_model__field="C",
            then="related_model__pk")
        ))
    )

Now there are lists of None or pk under each field_a, field_b and field_c for every object in queryset. You can also define other default value for Case instead of None.