Model_Auth_User::unique_key PHP Method

    public function unique_key($value)
    {
        return Validate::email($value) ? 'email' : 'username';
    }

	/**
	 * Allows a model to use a user id, in addition to email and username, as unique identifier.
	 *
	 * @param   mixed   unique value
	 * @return  string  field name
	 */
	public function unique_key($value)
	{
		return (is_int($value)) ? 'id' : parent::unique_key($value);
	}