Changelist view does not use select_related() on a nullable foreign key

[mrts]

Given the following models.py:

from django.db import models

class Base(models.Model):
    name = models.CharField(max_length=10)
    lots_of_text = models.TextField()

    class Meta:
        abstract = True

    def __unicode__(self):
        return self.name

class A(Base):
    a_field = models.CharField(max_length=10)

class B(Base):
    b_field = models.CharField(max_length=10)

class C(Base):
    a = models.ForeignKey(A)
    b = models.ForeignKey(B)
    is_published = models.BooleanField()

and the following admin.py:

from django.contrib import admin
from improve_admin.models import A, B, C

class CAdmin(admin.ModelAdmin):
    list_display = ('name', 'a', 'b', 'is_published')

admin.site.register(A)
admin.site.register(B)
admin.site.register(C, CAdmin)

select_related() is used as documented in ​http://docs.djangoproject.com/en/dev/ref/contrib/admin/#list-select-related , resulting in the following query:

SELECT
	"improve_admin_c"."id", "improve_admin_c"."name", "improve_admin_c"."lots_of_text", "improve_admin_c"."a_id", "improve_admin_c"."b_id", "improve_admin_c"."is_published", "improve_admin_a"."id", "improve_admin_a"."name", "improve_admin_a"."lots_of_text", "improve_admin_a"."a_field", "improve_admin_b"."id", "improve_admin_b"."name", "improve_admin_b"."lots_of_text", "improve_admin_b"."b_field"
FROM
	"improve_admin_c"
	INNER JOIN "improve_admin_a" ON ("improve_admin_c"."a_id" = "improve_admin_a"."id")
	INNER JOIN "improve_admin_b" ON ("improve_admin_c"."b_id" = "improve_admin_b"."id")
ORDER BY
	"improve_admin_c"."id" DESC

---

Setting one foreign key field to be nullable:

# diff between the original and new models.py
 class C(Base):
-    a = models.ForeignKey(A)
+    a = models.ForeignKey(A, blank=True, null=True)
     b = models.ForeignKey(B)

results in the following SQL:

SELECT
	"improve_admin_c"."id", "improve_admin_c"."name", "improve_admin_c"."lots_of_text", "improve_admin_c"."a_id", "improve_admin_c"."b_id", "improve_admin_c"."is_published", "improve_admin_b"."id", "improve_admin_b"."name", "improve_admin_b"."lots_of_text", "improve_admin_b"."b_field"
FROM
	"improve_admin_c"
	INNER JOIN "improve_admin_b" ON ("improve_admin_c"."b_id" = "improve_admin_b"."id")
ORDER BY
	"improve_admin_c"."id" DESC

and additional n queries for each of the referred a fields:

SELECT
	"improve_admin_a"."id", "improve_admin_a"."name", "improve_admin_a"."lots_of_text", "improve_admin_a"."a_field"
FROM
	"improve_admin_a"
WHERE
	"improve_admin_a"."id" = 2

[Karen Tracey]

Admin doesn't introspect the model and decide not to use select_related() if null=True on a ForeignKey. Rather, select_related() itself does not follow null=True ForeignKeys, as documented: ​http://docs.djangoproject.com/en/dev/ref/models/querysets/#select-related

"Note that, by default, select_related() does not follow foreign keys that have null=True."

This strikes me as another case where if you want to control the queries the admin is going to issue to this level of detail, you'll need to specify the ModelAdmin queryset() yourself.

[mrts]

Thanks for pointing that out!

[mrts]

See #10742