The model / model manager layer is the right place at which to design your state changing API. REST framework absolutely you to work with the grain there. Some good practice I'd recommend...

* Write the `create()` and/or `update()` methods explicitly on the serializer class.

* Push logic into the model and model manager where possible and only have the serializer `.save()` as a thin layer on top of that.

That way a serializer class still has all the behavior it needs to map both ways between persisted objects and their corresponding native python representations, but you still have a well separated model API.