如何使 Laravel 的 Validator $rules 成为可选?

Let's say I have User model with two methods:

User.php

class User extends Eloquent
{  
    /* Validation rules */
    private static $rules = array(
        'user'  => 'unique:users|required|alpha_num',
        'email' => 'required|email'
    );

    /* Validate against registration form */
    public static function register($data)
    {
        $validator = Validator::make($data, static::$rules);
        if($validator->fails())
        {
            /*... do someting */
        }
        else
        {
            /* .. do something else */
        }
    }

    /* Validate against update form */
    public static function update($data)
    {
        $validator = Validator::make($data, static::$rules);
        if($validator->fails())
        {
            /*... do someting */
        }
        else
        {
            /* .. do something else */
        }
    }
}

我的问题:我怎样才能让验证规则成为可选的，所以即使 update() 的数据只是 email 字段，它会忽略 user 并仍然验证为 true.
这是可能的还是我遗漏了什么?

'user'  => 'unique:users|alpha_num',

'user'  => 'unique:users|required|alpha_num',

private function getValidationRules($rules)
{
    if ($rules == UPDATE_EMAIL)
    {
        return array('email' => 'required|email');
    } else {
        return array(
            'user'  => 'unique:users|required|alpha_num',
            'email' => 'required|email'
        );
    }
}