export function changePassword(userId, oldPass, newPass) {
  var query = User.findById(userId);
  return query.exec()
    .then(user => {
      if (user.authenticate(oldPass)) {
        user.password = newPass;
        return user.save();
      } else {
        // I want to test that this Exception is thrown
        throw new UserNotAuthenticatedError();
      }
    });
}

describe('#changePassword', function() {
    it('should throw a UserNotAuthenticatedError when passing wrong password', function() {
      var userId = user._id;
      var wrongPwd = 'wrongpassword';
      var newPwd = 'new password';
      // so far the following only tells me that the Promise was rejected, but
      // I WANT TO TEST THAT THE REJECTION WAS DUE TO A 'UserNotAuthenticatedError'
      UserService.changePassword(userId, wrongPwd, newPwd).should.be.rejected;
    });
  });