docs: def on_get(self, req, resp, rid, related):
          """ Find the related model & serialize it back
  
          If the parent resource of the related model doesn't
          exist then abort on a 404.
          """
  
          signals.pre_req.send(self.model)
          signals.pre_req_find.send(self.model)
  
          if not hasattr(self.model, related):
              abort(InvalidURL(**{
                  'detail': 'The "%s" resource does not have a related '
                            'resource named "%s". This is an error, check '
                            'your spelling & retry.' % (self.rtype, related)
              }))
  
          model = find(self.model, rid)
          try:
              model_related = getattr(model, related).load()
          except AttributeError:
              model_related = None
  
          if isinstance(model_related, list):
              props = to_rest_models(model_related, includes=req.includes)
          elif model:
              props = to_rest_model(model_related, includes=req.includes)
          else:
              props = model_related
  
          resp.serialize(props)
  
          signals.post_req.send(self.model)
          signals.post_req_find.send(self.model)