Ticket #25371: migration_reverse.patch

docs/ref/migration-operations.txt

@@ -224 +224 @@
 If you want to include literal percent signs in the query, you have to double
 them if you are passing parameters.
 
+The ``reverse_sql`` statement is executed when the migration is reversed. So you can
+reverse the changes done in the first statement::
+
+    migrations.RunSQL(
+        ["INSERT INTO musician (name) VALUES (%s);", ['Reinhardt']],
+        ["DELETE FROM musician where name=%s;", ['Reinhardt']]
+    )
+
 The ``state_operations`` argument is so you can supply operations that are
 equivalent to the SQL in terms of project state; for example, if you are
 manually creating a column, you should pass in a list containing an ``AddField``
@@ -265 +273 @@
 instance of :class:`SchemaEditor
 <django.db.backends.base.schema.BaseDatabaseSchemaEditor>`.
 
+The ``reverse_code`` argument is called when running migrations backward.
+This callable is used so that the migration is reversible by undoing
+what is being done in the ``code`` argument.
+
 The optional ``hints`` argument will be passed as ``**hints`` to the
 :meth:`allow_migrate` method of database routers to assist them in making a
 routing decision. See :ref:`topics-db-multi-db-hints` for more details on
@@ -294 +306 @@
             Country(name="France", code="fr"),
         ])
 
+    def backwards_func(apps, schema_editor):
+        # forwards_func creates two Country instance,
+        # so backwards_func should delete them
+        Country = apps.get_model("myapp", "Country")
+        db_alias = schema_editor.connection.alias
+        Country.objects.using(db_alias).filter(name="USA", code="us").delete()
+        Country.objects.using(db_alias).filter(name="France", code="fr").delete()
+
     class Migration(migrations.Migration):
 
         dependencies = []
@@ -301 +321 @@
         operations = [
             migrations.RunPython(
                 forwards_func,
+                backwards_func
             ),
         ]