Made MigrationWriter look for a "deconstruct" attribute on functions.

Refs #20978.

django/db/migrations/writer.py

@@ -73,6 +73,26 @@ class MigrationWriter(object):
                 raise ImportError("Cannot open migrations module %s for app %s" % (migrations_module_name, self.migration.app_label))
         return os.path.join(basedir, self.filename)
 
+    @classmethod
+    def serialize_deconstructed(cls, path, args, kwargs):
+        module, name = path.rsplit(".", 1)
+        if module == "django.db.models":
+            imports = set(["from django.db import models"])
+            name = "models.%s" % name
+        else:
+            imports = set(["import %s" % module])
+            name = path
+        arg_strings = []
+        for arg in args:
+            arg_string, arg_imports = cls.serialize(arg)
+            arg_strings.append(arg_string)
+            imports.update(arg_imports)
+        for kw, arg in kwargs.items():
+            arg_string, arg_imports = cls.serialize(arg)
+            imports.update(arg_imports)
+            arg_strings.append("%s=%s" % (kw, arg_string))
+        return "%s(%s)" % (name, ", ".join(arg_strings)), imports
+
     @classmethod
     def serialize(cls, value):
         """
@@ -119,23 +139,7 @@ class MigrationWriter(object):
         # Django fields
         elif isinstance(value, models.Field):
             attr_name, path, args, kwargs = value.deconstruct()
-            module, name = path.rsplit(".", 1)
+            return cls.serialize_deconstructed(path, args, kwargs)
-            if module == "django.db.models":
-                imports = set(["from django.db import models"])
-                name = "models.%s" % name
-            else:
-                imports = set(["import %s" % module])
-                name = path
-            arg_strings = []
-            for arg in args:
-                arg_string, arg_imports = cls.serialize(arg)
-                arg_strings.append(arg_string)
-                imports.update(arg_imports)
-            for kw, arg in kwargs.items():
-                arg_string, arg_imports = cls.serialize(arg)
-                imports.update(arg_imports)
-                arg_strings.append("%s=%s" % (kw, arg_string))
-            return "%s(%s)" % (name, ", ".join(arg_strings)), imports
         # Functions
         elif isinstance(value, (types.FunctionType, types.BuiltinFunctionType)):
             # Special-cases, as these don't have im_class
@@ -152,6 +156,8 @@ class MigrationWriter(object):
                 klass = value.im_class
                 module = klass.__module__
                 return "%s.%s.%s" % (module, klass.__name__, value.__name__), set(["import %s" % module])
+            elif hasattr(value, 'deconstruct'):
+                return cls.serialize_deconstructed(*value.deconstruct())
             elif value.__name__ == '<lambda>':
                 raise ValueError("Cannot serialize function: lambda")
             elif value.__module__ is None:

django/db/models/deletion.py

@@ -35,6 +35,7 @@ def SET(value):
     else:
         def set_on_delete(collector, field, sub_objs, using):
             collector.add_field_update(field, value, sub_objs)
+    set_on_delete.deconstruct = lambda: ('django.db.models.SET', (value,), {})
     return set_on_delete
 
 

tests/migrations/test_writer.py

@@ -63,6 +63,10 @@ class WriterTests(TestCase):
         # Functions
         with six.assertRaisesRegex(self, ValueError, 'Cannot serialize function: lambda'):
             self.assertSerializedEqual(lambda x: 42)
+        self.assertSerializedEqual(models.SET_NULL)
+        string, imports = MigrationWriter.serialize(models.SET(42))
+        self.assertEqual(string, 'models.SET(42)')
+        self.serialize_round_trip(models.SET(42))
         # Datetime stuff
         self.assertSerializedEqual(datetime.datetime.utcnow())
         self.assertSerializedEqual(datetime.datetime.utcnow)