Fixed #30033 -- Conformed to the recommended table alterations procedure on SQlite3.

Refs #29182.

The previous implementation was following a procedure explicitly documented
as incorrect and was the origin of the breakage experienced on SQLite 3.26
release that were addressed by c8ffdbe514.

Thanks to Richard Hipp for pointing out the usage of the incorrect procedure.

django/db/backends/sqlite3/schema.py

@@ -1,4 +1,3 @@
-import contextlib
 import copy
 from decimal import Decimal
 
@@ -33,6 +32,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
 
     def __exit__(self, exc_type, exc_value, traceback):
         super().__exit__(exc_type, exc_value, traceback)
+        self.connection.check_constraints()
         self.connection.cursor().execute('PRAGMA legacy_alter_table = OFF')
         self.connection.enable_constraint_checking()
 
@@ -139,11 +139,17 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         """
         Shortcut to transform a model from old_model into new_model
 
+        This follows the correct procedure to perform non-rename or column
+        addition operations based on SQLite's documentation
+
+        https://www.sqlite.org/lang_altertable.html#caution
+
         The essential steps are:
-          1. rename the model's existing table, e.g. "app_model" to "app_model__old"
-          2. create a table with the updated definition called "app_model"
-          3. copy the data from the old renamed table to the new table
-          4. delete the "app_model__old" table
+          1. Create a table with the updated definition called "new__app_model"
+          2. Copy the data from the existing "app_model" table to the new table
+          3. Drop the "app_model" table
+          4. Rename the "new__app_model" table to "app_model"
+          5. Restore any index of the previous "app_model" table.
         """
         # Self-referential fields must be recreated rather than copied from
         # the old model to ensure their remote_field.field_name doesn't refer
@@ -205,11 +211,6 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         # Work inside a new app registry
         apps = Apps()
 
-        # Provide isolated instances of the fields to the new model body so
-        # that the existing model's internals aren't interfered with when
-        # the dummy model is constructed.
-        body = copy.deepcopy(body)
-
         # Work out the new value of unique_together, taking renames into
         # account
         unique_together = [
@@ -233,7 +234,16 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
 
         constraints = list(model._meta.constraints)
 
-        # Construct a new model for the new state
+        # Provide isolated instances of the fields to the new model body so
+        # that the existing model's internals aren't interfered with when
+        # the dummy model is constructed.
+        body_copy = copy.deepcopy(body)
+
+        # Construct a new model with the new fields to allow self referential
+        # primary key to resolve to. This model won't ever be materialized as a
+        # table and solely exists for foreign key reference resolution purposes.
+        # This wouldn't be required if the schema editor was operating on model
+        # states instead of rendered models.
         meta_contents = {
             'app_label': model._meta.app_label,
             'db_table': model._meta.db_table,
@@ -244,42 +254,46 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
             'apps': apps,
         }
         meta = type("Meta", (), meta_contents)
-        body['Meta'] = meta
-        body['__module__'] = model.__module__
+        body_copy['Meta'] = meta
+        body_copy['__module__'] = model.__module__
+        type(model._meta.object_name, model.__bases__, body_copy)
 
-        temp_model = type(model._meta.object_name, model.__bases__, body)
+        # Construct a model with a renamed table name.
+        body_copy = copy.deepcopy(body)
+        meta_contents = {
+            'app_label': model._meta.app_label,
+            'db_table': 'new__%s' % model._meta.db_table,
+            'unique_together': unique_together,
+            'index_together': index_together,
+            'indexes': indexes,
+            'constraints': constraints,
+            'apps': apps,
+        }
+        meta = type("Meta", (), meta_contents)
+        body_copy['Meta'] = meta
+        body_copy['__module__'] = model.__module__
+        new_model = type('New%s' % model._meta.object_name, model.__bases__, body_copy)
 
-        # We need to modify model._meta.db_table, but everything explodes
-        # if the change isn't reversed before the end of this method. This
-        # context manager helps us avoid that situation.
-        @contextlib.contextmanager
-        def altered_table_name(model, temporary_table_name):
-            original_table_name = model._meta.db_table
-            model._meta.db_table = temporary_table_name
-            yield
-            model._meta.db_table = original_table_name
-
-        with altered_table_name(model, model._meta.db_table + "__old"):
-            # Rename the old table to make way for the new
-            self.alter_db_table(
-                model, temp_model._meta.db_table, model._meta.db_table,
-                disable_constraints=False,
-            )
         # Create a new table with the updated schema.
-            self.create_model(temp_model)
+        self.create_model(new_model)
 
         # Copy data from the old table into the new table
-            field_maps = list(mapping.items())
         self.execute("INSERT INTO %s (%s) SELECT %s FROM %s" % (
-                self.quote_name(temp_model._meta.db_table),
-                ', '.join(self.quote_name(x) for x, y in field_maps),
-                ', '.join(y for x, y in field_maps),
+            self.quote_name(new_model._meta.db_table),
+            ', '.join(self.quote_name(x) for x in mapping),
+            ', '.join(mapping.values()),
             self.quote_name(model._meta.db_table),
         ))
 
-            # Delete the old table
+        # Delete the old table to make way for the new
         self.delete_model(model, handle_autom2m=False)
 
+        # Rename the new table to take way for the old
+        self.alter_db_table(
+            new_model, new_model._meta.db_table, model._meta.db_table,
+            disable_constraints=False,
+        )
+
         # Run deferred SQL on correct table
         for sql in self.deferred_sql:
             self.execute(sql)