[5.2.x] Fixed #36373 -- Fixed select_related() crash on foreign object for a composite pk.

Thanks Jacob Walls for the report and Sarah for the in-depth review.

Backport of 8be0c0d690 from main.

django/db/models/query.py

@@ -2715,7 +2715,11 @@ class RelatedPopulator:
             )
 
         self.model_cls = klass_info["model"]
-        self.pk_idx = self.init_list.index(self.model_cls._meta.pk.attname)
+        # A primary key must have all of its constituents not-NULL as
+        # NULL != NULL and thus NULL cannot be referenced through a foreign
+        # relationship. Therefore checking for a single member of the primary
+        # key is enough to determine if the referenced object exists or not.
+        self.pk_idx = self.init_list.index(self.model_cls._meta.pk_fields[0].attname)
         self.related_populators = get_related_populators(klass_info, select, self.db)
         self.local_setter = klass_info["local_setter"]
         self.remote_setter = klass_info["remote_setter"]

docs/releases/5.2.2.txt

@@ -9,4 +9,5 @@ Django 5.2.2 fixes several bugs in 5.2.1.
 Bugfixes
 ========
 
-* ...
+* Fixed a crash when using ``select_related`` against a ``ForeignObject``
+  originating from a model with a ``CompositePrimaryKey`` (:ticket:`36373`).

tests/composite_pk/models/tenant.py

@@ -14,17 +14,18 @@ class Token(models.Model):
     secret = models.CharField(max_length=10, default="", blank=True)
 
 
-class BaseModel(models.Model):
+class AbstractUser(models.Model):
     pk = models.CompositePrimaryKey("tenant_id", "id")
     tenant = models.ForeignKey(Tenant, on_delete=models.CASCADE)
+    email = models.EmailField(unique=True)
     id = models.SmallIntegerField(unique=True)
 
     class Meta:
         abstract = True
 
 
-class User(BaseModel):
-    email = models.EmailField(unique=True)
+class User(AbstractUser):
+    pass
 
 
 class Comment(models.Model):
@@ -35,13 +36,14 @@ class Comment(models.Model):
         related_name="comments",
     )
     id = models.SmallIntegerField(unique=True, db_column="comment_id")
-    user_id = models.SmallIntegerField()
+    user_id = models.SmallIntegerField(null=True)
     user = models.ForeignObject(
         User,
         on_delete=models.CASCADE,
         from_fields=("tenant_id", "user_id"),
         to_fields=("tenant_id", "id"),
         related_name="comments",
+        null=True,
     )
     text = models.TextField(default="", blank=True)
     integer = models.IntegerField(default=0)

tests/composite_pk/tests.py

@@ -170,6 +170,14 @@ class CompositePKTests(TestCase):
         with self.assertNumQueries(1):
             self.assertEqual(user.email, self.user.email)
 
+    def test_select_related(self):
+        Comment.objects.create(tenant=self.tenant, id=2)
+        with self.assertNumQueries(1):
+            comments = list(Comment.objects.select_related("user").order_by("pk"))
+            self.assertEqual(len(comments), 2)
+            self.assertEqual(comments[0].user, self.user)
+            self.assertIsNone(comments[1].user)
+
     def test_model_forms(self):
         fields = ["tenant", "id", "user_id", "text", "integer"]
         self.assertEqual(list(CommentForm.base_fields), fields)