如何使用'select_related'接收来自related（ForeignKey）Django模型的所有字段

Question

Env:

python >= 3.6
Django >=2.1.0

Models example:

class A(models.Model):
    id = models.AutoField(primary_key=True)
    a_field = models.CharField(max_length=256, default="example")


class B(models.Model):
    id = models.AutoField(primary_key=True)
    b_field = models.CharField(max_length=256, default="example")
    a = models.ForeignKey(A, related_name="b", on_delete=models.CASCADE)

Question:

How I can fetch only required fields from related models using select_related()

How it could be done with prefetch_related :

from django.db.models import Prefetch
prefetch_obj = Prefetch("a", queryset=A.objects.only("id", "a_field"))
B.objects.only("id", "b_field", "a").prefetch_related(prefetch_obj)

But it produces 2 request to DB due to prefetch_related using.

SELECT `main_b`.`id`, `main_b`.`b_field`, `main_b`.`a_id` FROM `main_b`  LIMIT 21; args=()
SELECT `main_a`.`id`, `main_a`.`a_field` FROM `main_a` WHERE `main_a`.`id` IN (1); args=(1,)

If I use select_related it makes 1 DB call, but fetches all fields from A model:

models.B.objects.only("id", "b_field", "a").select_related("a")

SELECT `main_b`.`id`, `main_b`.`b_field`, `main_b`.`a_id`, `main_a`.`id`, `main_a`.`a_field`, `main_a`.`a_not_required_field` FROM `main_b` INNER JOIN `main_a` ON (`main_b`.`a_id` = `main_a`.`id`)  LIMIT 21; args=()

Answer 1

You can use .select_related('related_obj').only('related_obj__field') . Please see the docs on Only .