get(id) {
    return this.findById(id)
      .exec()
      .then((user) => {
        if (user) {
          return user;
        }
        const err = new APIError('No such user exists!', httpStatus.NOT_FOUND);
        return Promise.reject(err);
      });
  }
Get user
@param {ObjectId} id - The objectId of user.
@returns {Promise<User, APIError>}