Resolve "Provide infrastructure for live documents"

Closes #3 (closed)

aleksis/apps/resint/models.py

@@ -202,7 +202,7 @@ class LiveDocument(ExtensiblePolymorphicModel):
 
         Has to be implemented by subclasses.
         """
-        pass
+        raise NotImplementedError("Subclasses of LiveDocument must implement update()")
 
     def __str__(self) -> str:
         return self.name