Laravel 4：自定义验证规则

I've an owners table with a user_id field, this field can be NULL. because not every owner may have an user account. I need to check if the user_id field is NULL on the owners table, before creating a user and updating that table. How do I do it with Laravel's Validation ?

You can extend the validator, as per the docs

Validator::extend('foo', function($attribute, $value, $parameters)
{
    return $value == NULL;
});

See the custom validation section http://four.laravel.com/docs/validation