docs: function titleField (modelName, field, callback) {
  
      // Make this a no-op unless the field being evaluated is `'title'`.
      if (field !== 'title') {
          return callback(null, field);
      }
  
      if (get(modelName, true).schema.paths[field]) {
          return callback(null, field);
      }
  
      model(modelName, (err, opts) => {
  
          if (err) {
              return callback(err);
          }
  
          return callback(null, opts.title);
  
      });
  
  }