I believe the rule is applied once you set the value, not when you're saving it.
I had a similar issue - the filter wasn't working if I didn't assign any value to the field. I've written my own save method:
public function save(Validation $validation = NULL)
{
if (!$this->loaded())
{
$this->ordering = 0;
}
return parent::save($validation);
}
this way the ordering would always be assigned for newly created objects and my filter would work.
And that's how I built another model. It's a company model that has a unique company name. Rules for the field are defined like this:
'name' => array(
array('not_empty'),
array('max_length', array(':value', 255)),
array(array($this, 'unique_name'))
)
And I have a method:
public function unique_name($value)
{
$exists = (bool) DB::select(array(DB::expr('COUNT(*)'), 'total_count'))
->from($this->_table_name)
->where('name', '=', $value)
->where($this->_primary_key, '!=', $this->pk())
->execute($this->_db)
->get('total_count');
return !$exists;
}
It basically checks if there are any other companies with the same name as the current one. Maybe this will give you the idea of what could be wrong with your solution.