Unexpected query result after call QuerySet.only() to ForeignKey() with 'self' reference

[Mahardhika Praja]

Unexpected query result after calling QuerySet.only('ref__field') to ForeignKey('self')

class MyModel(models.Model):
    f1 = models.IntegerField()
    f2 = models.IntegerField()
    f3 = models.IntegerField()
    ref = models.ForeignKey("self", on_delete=models.CASCADE)

print(str(MyModel.objects.select_related('ref').only('ref__f1').query))

Output

'SELECT "mymodel"."id", "mymodel"."f1", "mymodel"."ref_id", T2."id", T2."f1", T2."ref_id" FROM "mymodel" INNER JOIN "mymodel" T2 ON ("mymodel"."ref_id" = T2."id")'

Fields "mymodel"."f2" and "mymodel"."f3" are excluded from column selection

Another try

print(str(MyModel.objects.select_related('ref').only('f1', 'f2', 'f3', 'ref__f1').query))

Output

'SELECT "mymodel"."id", "mymodel"."f1", "mymodel"."f2", "mymodel"."f3", "mymodel"."ref_id", T2."id", T2."f1", T2."f2", T2."f3", T2."ref_id" FROM "mymodel" INNER JOIN "mymodel" T2 ON ("mymodel"."ref_id" = T2."id")'

[Mariusz Felisiak]

only() works as ​documented and expected, i.e. it replaces the set of fields to load immediately, so in your case all columns "mymodel"."f1", "mymodel"."f2", and "mymodel"."f3" from MyModel should be omitted in the SELECT clause. f1 appears because MyModel is involved more than once, it was fixed in b3db6c8dcb5145f7d45eff517bcd96460475c879 (see #21204).