Note removal of 'save_object' and fail loudly if it exists

docs/topics/3.0-announcement.md

@@ -84,6 +84,8 @@ The resulting API changes are further detailed below.
 
 The `.restore_object()` method is now replaced with two separate methods, `.create()` and `.update()`.
 
+These methods also replace the optional `.save_object()` method, which no longer exists.
+
 When using the `.create()` and `.update()` methods you should both create *and save* the object instance. This is in contrast to the previous `.restore_object()` behavior that would instantiate the object but not save it.
 
 The following example from the tutorial previously used `restore_object()` to handle both creating and updating object instances.

rest_framework/serializers.py

@@ -108,6 +108,13 @@ class BaseSerializer(Field):
         raise NotImplementedError('`create()` must be implemented.')
 
     def save(self, **kwargs):
+        assert not hasattr(self, 'save_object'), (
+            'Serializer `%s.%s` has old-style version 2 `.save_object()` '
+            'that is no longer compatible with REST framework 3. '
+            'Use the new-style `.create()` and `.update()` methods instead.' %
+            (self.__class__.__module__, self.__class__.__name__)
+        )
+
         validated_data = dict(
             list(self.validated_data.items()) +
             list(kwargs.items())