Annotate each query with a function of itself

Question

I need to annotate each query in queryset using a model method:

class myModel(models.Model):
    ...

    def myModel_foo(self):
        ....
        return myModel_foo

I need something like .annotate(myModel_foo=myModel.myModel_foo()). The problem is myModel_foo() requires self. I tried iterating over queryset and using query.annotate(myModel_foo=myModel.myModel_foo(self)), but i got object has no attribute 'annotate' error. What's the correct approach?

UPDATE

OK, the idea is this: i have two models with single to one relation.

class myModel1(models.Model):
  fk = ForeignKey(myModel2)
  status = ChoiceField

class myModel2(models.Model):

  def get_status(self):
    # query all objects from myModel1, get status of the last one and 
    # return it
    return get_status

Then I want to send a queryset with status included in it via ajax call.

qs = MyModel2.objects.all()
qs_json = serializers.serialize('json', qs)
        return HttpResponse(qs_json, content_type='application/json')

How can i access it in template then in a way like qs_json[0].get_status?

You can not annotate with a function call, unless you call the function, and the result is an expression like Sum('field') for example. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Jun 3, 2020 at 20:27
Perhaps it is more helpful if you define a concrete example. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Jun 3, 2020 at 20:34

ADR · Accepted Answer · 2020-06-04 19:28:23Z

1

Just add a property to the model and don't use the "annotation" at all. like:

class MyModel(models.Model):
    @property
    def foo(self):
        ....
        return self.bar()

But be careful with N+1 queries problem if the bar makes extra queries. You can describe your particular case and I will help you with this.

UPDATE:

There still unclear is there extra queries or not. If not, you can still use the property. You just need the right serializer like this:

from rest_framework import serializers


class MyModelSerializer(serializers.ModelSerializer):
    foo = serializers.CharField(read_only=True)

    class Meta:
        model = MyModel
        fields = [
            # other fields
            'foo'
        ]

return Response(MyModelSerializer(MyModel.objects.all(), many=True).data)

edited Jun 4, 2020 at 19:28

answered Jun 3, 2020 at 20:39

ADR

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Viktor Over a year ago

I described it in main post

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