Annotate in django object using function

Question

I have two models Parent, Child

class Parent(models.Model):
    id = models.IntegerField(...)

class Child(models.Model)
    id = models.IntegerField(...)
    parent = models.ForeignKey(Parent, ...)
    wanted = models.CharField(default="yes")

I have a queryset

parents = Parent.objects.all()

I want to annotate a char field to parents queryset which is based on Child model

get_status(obj):
    # complex function which returns string based on child onjects
    children = Child.objects.filter(parent_id = obj.id)
    if children.count() == 1:
        return 'some'
    if children.count() == 2:
        return  'thing'

I want something like this, How to send object to get_status() function.

queryset.annotate(status = Value(get_status(), output_field=CharField()))

Iain Shelvington · Accepted Answer · 2021-06-18 07:54:58Z

1

from django.db.models import Count, Case, When, Value, CharField

Parent.objects.annotate(
    children=Count('child')
).annotate(
    status=Case(
        When(children__gte=2, then=Value('thing')),
        When(children=1, then=Value('some')),
        output_field=CharField()
    )
)

answered Jun 18, 2021 at 7:54

Iain Shelvington

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