Call function in model before validation

Hi guys,
I have a model with the following validation rule.

PHP Code:

'slug'  => 'is_unique[products.slug,ID,{ID}]|alpha_dash' 


I would like to automatically generate a slug when none is provided, so I added:

PHP Code:

protected $beforeInsert = ['generateSlug'];

protected function generateSlug(array $data)
    {
        if (empty($data['data']['slug']))
        {
            $data['data']['slug'] = url_title($data['data']['name'], '-', true);
        }

        return $data;
    } 


The problem seems that validation rules are evaluated before `beforeInsert` callback is called.

Could you help me?
Thanks.