code: def get_existing_model(model_name):
      
      try:
          model_cls = engine.get_document_cls(model_name)
          log.debug('Model `{}` already exists. Using existing one'.format(
              model_name))
          return model_cls
      except ValueError:
          log.debug('Model `{}` does not exist'.format(model_name))
docs: Try to find existing model class named `model_name`.
  
      :param model_name: String name of the model class.