Django：如何通过related_field中的特定条目来order_by？

Question

I have a typical ForeignKey relationship in Django:

class SchoolYear(models.Model):
    pass

class CycleHeader(models.Model):
    schoolyear = models.ForeignKey(SchoolYear, related_name="cycles")
    # Index is D, or A1-A4
    index = models.CharField(max_length=2)
    value = models.IntegerField()

Every SchoolYear may have up to 5 CycleHeaders. If there is any CycleHeader at all, there is a D record. Sometimes there are also records for A1-A4. I need to sort the SchoolYear records by the value field of the D record.

If I use schoolyears.order_by('cycles_ index', 'cycles _value'), the results vary by how many CycleHeader records there are. What I'd like to do is annotate the schoolyear records with either the D record or the value of the D record, and I can't seem to figure out a way to do that. Am I missing something?

Answer 1

Here's the solution I eventually came up with thanks to the hints:

    # schoolyears is a QuerySet based on earlier filtering

    schoolyears1 = schoolyears.filter(cycles__index='D').order_by('cycles__value', 'school__name')
    schoolyears2 = schoolyears.filter(cycles__isnull=True).order_by('school__name')
    schoolyears = schoolyears2 | schoolyears1