[1.11.x] Fixed #28298 -- Prevented a primary key alteration from adding a foreign key constraint if db_constraint=False.

Backport of fba0eaa5d6 from master

tests/schema/tests.py

@@ -9,9 +9,9 @@ from django.db import (
 from django.db.models import Model
 from django.db.models.deletion import CASCADE, PROTECT
 from django.db.models.fields import (
-    AutoField, BigIntegerField, BinaryField, BooleanField, CharField,
-    DateField, DateTimeField, IntegerField, PositiveIntegerField, SlugField,
-    TextField, TimeField,
+    AutoField, BigAutoField, BigIntegerField, BinaryField, BooleanField,
+    CharField, DateField, DateTimeField, IntegerField, PositiveIntegerField,
+    SlugField, TextField, TimeField,
 )
 from django.db.models.fields.related import (
     ForeignKey, ForeignObject, ManyToManyField, OneToOneField,
@@ -21,7 +21,7 @@ from django.db.transaction import TransactionManagementError, atomic
 from django.test import (
     TransactionTestCase, mock, skipIfDBFeature, skipUnlessDBFeature,
 )
-from django.test.utils import CaptureQueriesContext
+from django.test.utils import CaptureQueriesContext, isolate_apps
 from django.utils import timezone
 
 from .fields import (
@@ -287,6 +287,37 @@ class SchemaTests(TransactionTestCase):
             editor.alter_field(Author, new_field2, new_field, strict=True)
         self.assertForeignKeyNotExists(Author, 'tag_id', 'schema_tag')
 
+    @isolate_apps('schema')
+    def test_no_db_constraint_added_during_primary_key_change(self):
+        """
+        When a primary key that's pointed to by a ForeignKey with
+        db_constraint=False is altered, a foreign key constraint isn't added.
+        """
+        class Author(Model):
+            class Meta:
+                app_label = 'schema'
+
+        class BookWeak(Model):
+            author = ForeignKey(Author, CASCADE, db_constraint=False)
+
+            class Meta:
+                app_label = 'schema'
+
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+            editor.create_model(BookWeak)
+        self.assertForeignKeyNotExists(BookWeak, 'author_id', 'schema_author')
+        old_field = Author._meta.get_field('id')
+        new_field = BigAutoField(primary_key=True)
+        new_field.model = Author
+        new_field.set_attributes_from_name('id')
+        # @isolate_apps() and inner models are needed to have the model
+        # relations populated, otherwise this doesn't act as a regression test.
+        self.assertEqual(len(new_field.model._meta.related_objects), 1)
+        with connection.schema_editor() as editor:
+            editor.alter_field(Author, old_field, new_field, strict=True)
+        self.assertForeignKeyNotExists(BookWeak, 'author_id', 'schema_author')
+
     def _test_m2m_db_constraint(self, M2MFieldClass):
         class LocalAuthorWithM2M(Model):
             name = CharField(max_length=255)