Fixed #23091: CreateModel and AddField were clashing with deferred SQL

django/db/backends/schema.py

@@ -546,6 +546,7 @@ class BaseDatabaseSchemaEditor(object):
                     }
                 )
         # Drop any FK constraints, we'll remake them later
+        fks_dropped = set()
         if old_field.rel:
             fk_names = self._constraint_names(model, [old_field.column], foreign_key=True)
             if strict and len(fk_names) != 1:
@@ -555,6 +556,7 @@ class BaseDatabaseSchemaEditor(object):
                     old_field.column,
                 ))
             for fk_name in fk_names:
+                fks_dropped.add((old_field.column,))
                 self.execute(
                     self.sql_delete_fk % {
                         "table": self.quote_name(model._meta.db_table),
@@ -737,7 +739,7 @@ class BaseDatabaseSchemaEditor(object):
                 }
             )
         # Does it have a foreign key?
-        if new_field.rel:
+        if new_field.rel and fks_dropped:
             to_table = new_field.rel.to._meta.db_table
             to_column = new_field.rel.get_related_field().column
             self.execute(

tests/migrations/test_operations.py

@@ -1051,6 +1051,33 @@ class OperationTests(OperationTestBase):
             operation.database_backwards("test_alorwrtto", editor, new_state, project_state)
         self.assertColumnNotExists("test_alorwrtto_rider", "_order")
 
+    def test_alter_fk(self):
+        """
+        Tests that creating and then altering an FK works correctly
+        and deals with the pending SQL (#23091)
+        """
+        project_state = self.set_up_test_model("test_alfk")
+        # Test adding and then altering the FK in one go
+        create_operation = migrations.CreateModel(
+            name="Rider",
+            fields=[
+                ("id", models.AutoField(primary_key=True)),
+                ("pony", models.ForeignKey(to="Pony")),
+            ],
+        )
+        create_state = project_state.clone()
+        create_operation.state_forwards("test_alfk", create_state)
+        alter_operation = migrations.AlterField(
+            model_name='Rider',
+            name='pony',
+            field=models.ForeignKey(editable=False, to="Pony"),
+        )
+        alter_state = create_state.clone()
+        alter_operation.state_forwards("test_alfk", alter_state)
+        with connection.schema_editor() as editor:
+            create_operation.database_forwards("test_alfk", editor, project_state, create_state)
+            alter_operation.database_forwards("test_alfk", editor, create_state, alter_state)
+
     @unittest.skipIf(sqlparse is None and connection.features.requires_sqlparse_for_splitting, "Missing sqlparse")
     def test_run_sql(self):
         """

tests/schema/tests.py

@@ -331,6 +331,48 @@ class SchemaTests(TransactionTestCase):
         self.assertEqual(columns['name'][0], "TextField")
         self.assertEqual(bool(columns['name'][1][6]), False)
 
+    @unittest.skipUnless(connection.features.supports_foreign_keys, "No FK support")
+    def test_alter_fk(self):
+        """
+        Tests altering of FKs
+        """
+        # Create the table
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+            editor.create_model(Book)
+        # Ensure the field is right to begin with
+        columns = self.column_classes(Book)
+        self.assertEqual(columns['author_id'][0], "IntegerField")
+        # Make sure the FK constraint is present
+        constraints = self.get_constraints(Book._meta.db_table)
+        for name, details in constraints.items():
+            if details['columns'] == ["author_id"] and details['foreign_key']:
+                self.assertEqual(details['foreign_key'], ('schema_author', 'id'))
+                break
+        else:
+            self.fail("No FK constraint for author_id found")
+        # Alter the FK
+        new_field = ForeignKey(Author, editable=False)
+        new_field.set_attributes_from_name("author")
+        with connection.schema_editor() as editor:
+            editor.alter_field(
+                Book,
+                Book._meta.get_field_by_name("author")[0],
+                new_field,
+                strict=True,
+            )
+        # Ensure the field is right afterwards
+        columns = self.column_classes(Book)
+        self.assertEqual(columns['author_id'][0], "IntegerField")
+        # Make sure the FK constraint is present
+        constraints = self.get_constraints(Book._meta.db_table)
+        for name, details in constraints.items():
+            if details['columns'] == ["author_id"] and details['foreign_key']:
+                self.assertEqual(details['foreign_key'], ('schema_author', 'id'))
+                break
+        else:
+            self.fail("No FK constraint for author_id found")
+
     def test_rename(self):
         """
         Tests simple altering of fields