How to annotate Django Queryset with the same value but with a different data type of one of existing fields?

Question

I have a Django model that has a NumberVal field which is models.FloatField().

Then I need to annotate the queryset objects like so .annotate(numberval_as_text=str(OuterRef("NumberVal"))) - the same value, but as a string except for the fact that this throws QuerySet.annotate() received non-expression(s): OuterRef(NumberVal) error which is clearly right but it demonstrates the point.

I need this because this annotation is followed by an extensive second annotation where I need this numberval_as_text in subquery filters.

Any hint would be appreciated.

Abdul Aziz Barkat · Accepted Answer · 2021-05-26 07:02:17Z

3

Calling str on an object obviously will give you a string representation of the object, hence you get the error. If one wants to type case something, they can use the Cast database function [Django docs]:

from django.db.models import CharField
from django.db.models.functions import Cast


queryset.annotate(numberval_as_text=Cast(OuterRef("NumberVal"), output_field=CharField(max_length=256)))

Note: Unless this annotation is in a subquery, you want to use F("NumberVal") (from django.db.models import F) instead of OuterRef("NumberVal")

answered May 26, 2021 at 7:02

Abdul Aziz Barkat

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Nafees Anwar · Accepted Answer · 2021-05-26 07:29:35Z

1

You don't need to use OuterRef. You can just use plain cast with annotation.

from django.db import models
from django.db.models.functions import Cast


qs.annotate(numberval_as_text=Cast('NumberVal', output_field=models.CharField()))

answered May 26, 2021 at 7:29

Nafees Anwar

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