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

Backport of fba0eaa5d60603721d7b4653e3efacbfb3613bd2 from master

django/db/backends/base/schema.py

@@ -774,7 +774,7 @@ class BaseDatabaseSchemaEditor(object):
         # Rebuild FKs that pointed to us if we previously had to drop them
         if old_field.primary_key and new_field.primary_key and old_type != new_type:
             for rel in new_field.model._meta.related_objects:
-                if not rel.many_to_many:
+                if not rel.many_to_many and rel.field.db_constraint:
                     self.execute(self._create_fk_sql(rel.related_model, rel.field, "_fk"))
         # Does it have check constraints we need to add?
         if old_db_params['check'] != new_db_params['check'] and new_db_params['check']:

docs/releases/1.11.3.txt

@@ -51,3 +51,6 @@ Bugfixes
   1.11.1, casting was added in Python to avoid localization of numeric values
   in Django templates, but this made some use cases more difficult. Casting is
   now done in the template using the ``|stringformat:'s'`` filter.
+
+* Prevented a primary key alteration from adding a foreign key constraint if
+  ``db_constraint=False`` (:ticket:`28298`).

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)