QuerySet.only() doesn't support annotations

[Kal Sze]

Example:

from django.db import models


class School(models.Model):
    full_name = models.TextField()


class Student(models.Model):
    full_name = models.TextField()
    date_of_birth = models.DateField()
    school = models.ForeignKey(School, on_delete=models.PROTECT)

And then:

from django.db.models import Subquery, OuterRef
from schools.models import School, Student

eldest_student_name = Student.objects.filter(school=OuterRef('id')).order_by('date_of_birth').values('full_name')[:1]

schools = School.objects.annotate(eldest_student_name=Subquery(eldest_student_name)).only('id', 'eldest_student_name')

for school in schools:
    # This line results in FieldDoesNotExist: School has no field named 'eldest_student_name'
    print(school.id, school.eldest_student_name)

Not sure if this is a bug or an (undocumented) limitation of only() (and maybe of defer(), by extension).

[Tim Graham]

As far as I tested, annotations don't need to be included in only() in order to be included in the query. The documentation for only() and defer() only discuss using it with fields, not with annotations.

[Kal Sze]

Replying to Tim Graham:

As far as I tested, annotations don't need to be included in only() in order to be included in the query. The documentation for only() and defer() only discuss using it with fields, not with annotations.

I know it doesn't need to be included, but I thought the point of only() and defer() is to exclude the fields that we are not interested in, when executing the initial database query. Having only() support annotations is useful if I'm only interested in the annotated field, and not in the regular fields.

[Tim Graham]

I'm not sure. Please test only() without arguments. If it works, it might be worth adding tests for (if not already in Django's tests) and documenting.

[Kal Sze]

Replying to Tim Graham:

I'm not sure. Please test only() without arguments. If it works, it might be worth adding tests for (if not already in Django's tests) and documenting.

So I did a quick TIAS and here is what I found out, by printing QuerySet.query:

• QuerySet.only() without any argument doesn't defer any field; it would select every field of the model plus any annotation added to the QuerySet;
• QuerySet.only('some_field') would select the id, some_field, plus any annotation added to the QuerySet.

Ideally, it would be great if only() behaved in one of two ways:

• without any argument, defer all fields except the id and annotations;

or

• accept annotated field names

But it might break old apps and it's too late to change.

[GitHub <noreply@…>]

In b126f69:

Refs #30052 -- Clarified that defer() and only() do not work with aggregated fields.

[Natalia <124304+nessita@…>]

In da92a971:

[4.2.x] Refs #30052 -- Clarified that defer() and only() do not work with aggregated fields.

Backport of b126f694160c4641e64e57dba6b022f06fbfa745 from main