Kohana 3: Validation rule for has_many through relationship - kohana

Question

Is it possible to create a validation rule in Kohana 3 that will validate the has_many through relationship?
Using the example on the guide page, a blog post can have many categories through the categories_posts table. Is there a validation rule that can be setup in the Post model to verify at least one category was added?
I tried the following:
public function rules()
{
return array(
'categories' => array(
array(array($this, 'has'), array('categories'))
)
);
}
because I see that the ORM::has function will return true/false. But I think because 'categories' is a relationship, and not a field, the rule I wrote never gets checked.
Any ideas?

Answer 1

You must save Post before adding has_many relations. You can check Post for categories after saving, and mark it as draft if they were not set.

Answer 2

Woo, good idea.
Focus in MVC design pattern. I think that's C business not the M.
if ($post->categories->find_all())
{
//blablabla
}

Answer 3

Since categories is external to the posts table, you'll want to use external validation. Create a function called Model_Post::rule_has_valid_categories($submitted_categories, $post) that returns a boolean denoting whether or not the submitted categories are valid for this post.
Then, create the extra rule just before you try to save the post:
$extra_rules = Validation::factory(array('categories' => $submitted_categories))
->rule(
'categories',
'Model_Post::rule_has_valid_categories',
array(':value', ':model')
);
try
{
$post->save($extra_rules);
}
catch (ORM_Validation_Exception $e)
{
// if categories rule failed, array will contain _external[categories] field
print_r($e->errors('models'));
}
You store the message in /application/messages/models/post/_external.php:
return array(
'categories' => array(
'Model_Post::rule_has_valid_categories' => 'Invalid categories'
),
);