docs: def _get_model(self, appname, modelname):
          """
          return model or None
          """
          app = self._get_app(appname)
          models = app.get_models()
          model = None
          for mod in models:
              if mod.__name__ == modelname:
                  model = mod
                  return model
          msg = "Model " + modelname + " not found"