code: def from_string(cls, model_id, default_project=None):
          
          proj, dset, model = _helpers._parse_3_part_id(
              model_id, default_project=default_project, property_name="model_id"
          )
          return cls.from_api_repr(
              {"projectId": proj, "datasetId": dset, "modelId": model}
          )
docs: Construct a model reference from model ID string.
  
          Args:
              model_id (str):
                  A model ID in standard SQL format. If ``default_project``
                  is not specified, this must included a project ID, dataset
                  ID, and model ID, each separated by ``.``.
              default_project (str):
                  Optional. The project ID to use when ``model_id`` does not
                  include a project ID.
  
          Returns:
              google.cloud.bigquery.model.ModelReference:
                  Model reference parsed from ``model_id``.
  
          Raises:
              ValueError:
                  If ``model_id`` is not a fully-qualified table ID in
                  standard SQL format.