Django Order By on Reverse Foreign Key value drops items without relationships

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Context

I am trying to sort a list of objects based on the value of a reverse foreign key relationship. I have partial solution, but filter used creates an inner join that drops objects without the relationship.

The outcome I want to create sorted queryset with all objects, threating "null" reverse relationships as if the related value was null.

Models

class Student(models.Model):
    name = models.CharField(max_length=128)


class Subject(models.Model):
    title = models.CharField(max_length=128)

class Grade(models.Model):
    student = models.ForeignKey("Student", related_name="grades", on_delete=models.CASCADE)
    subject = models.ForeignKey("Subject", related_name="grades", on_delete=models.CASCADE)
    value = models.IntegerField()

Fixtures

+------+------------------------+
| name | subject  | grade_value |
+------+----------+-------------+
| beth | math     | 100         |
| beth | history  | 100         |
| mark | math     | 90          |
| mark | history  | 90          |
| mike | history  | 80          |
+------+----------+-------------+

Desired Outcome

When sorting students by their "history" grade, I want to get [ beth (100), mark (90), mike (80) ]

Now let's say mark was sick at home and missed the math exam.

When sorting students by their "math" grade, I want to get [ beth (100), mark (90), mike (null) ]

Instead, for the sorted by math grade returns [ beth (100), mark (90) ].

Attempted Solution

I know I could fetch all and rejoin results, but I looking for an efficient ORM queryset solution if possible.

>>> Student.objects.filter(grades__subject__title="history").order_by("grades__value")
<Student: Beth>, <Student: Mark>, <Student: Mike>

>>> Student.objects.filter(grades__subject__title="math").order_by("grades__value")
<Student: Beth>, <Student: Mark>

Update

I tried using a union as suggested but I am not sure why it causes related look up to fail:

>>> Student.objects.filter(grades__subject__title="math")\
     .union(Student.objects.all())\
     .order_by("grades__value")

...
django.core.exceptions.FieldError: Cannot resolve keyword 'value' into field. Choices are: grades, id, name

Solution

Validation below using solution by @schillingt.

grades = Grade.objects.filter(
    student_id=OuterRef('id'), subject__title="math"
)
students = Student.objects.annotate(
    subject_grade=Subquery(grades.values('value')[:1])
)

>>> [f"{s.name}: {s.subject_grade}" for s in students.order_by("subject_grade")]
['mike: None', 'mark: 90', 'beth: 100']
>>>
>>> [f"{s.name}: {s.subject_grade}" for s in students.order_by("-subject_grade")]
['beth: 100', 'mark: 90', 'mike: None']
>>>
django-orm django-models django python
1 Answers

I would use a Subquery expression in an annotation.

from django.db.models import Subquery, OuterRef
subject = "math"
grades = Grade.objects.filter(
    student_id=OuterRef('id'),
    subject__title=subject,
).order_by('value')
students = Student.objects.annotate(
    annotated_grade=Subquery(grades.values('value')[:1]),
)
print(students.first().annotated_grade) # The first student's highest math grade.
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