distinct(field, query) {
    if (!field) return Promise.reject(new errors.InvalidArugmentError('field is required'));
    query = query || {};

    return Promise.fromCallback(callback => {
      this._dbCollection.distinct(field, query, callback);
    });
  }
@param {String} field - mongo field, including dot-notated fields
@param {Object} [query] - mongo query