Restricting a category for a certain country in Prestashop 1.5 - prestashop

Question

I need to restrict a category to a set of countries in Prestashop 1.5.
This restriction would prevent the shipping of a product belonging to such a category; as such, the users would still be able to see the products but they would not be able to buy them.
Ideally, I wanted to develop a module that would insert a list of countries (checkbox style, as in the Modules -> Payment page (AdminPayment)) inside a category's edit page, but I haven't been able to do so.
Why can't i simply paste the following code inside the renderForm() function?
Only the description is visible if i do so...
array(
'items' =>Country::getCountries(Context::getContext()->language->id),
'title' => $this->l('Country restrictions'),
'desc' => $this->l('Please mark the checkbox(es) for the country or countries for which you want to block the shipping.'),
'name_id' => 'country',
'identifier' => 'id_country',
'icon' => 'world',
),
EDIT:
I managed to get the list of countries working:
array(
'type' => 'checkbox',
'label' => $this->l('Restricted Countries').':',
'class' => 'sel_country',
'name' => 'restricted_countries',
'values' => array(
'query' => Country::getCountries(Context::getContext()->language->id),
'id' => 'id_country',
'name' => 'name'
),
'desc' => $this->l('Mark all the countries you want to block the selling to. The restrictions will always be applied to every subcategory as well')
),
Now, I can save these values by checking if the value "submitAddcategory" is being submitted in the postProcess function and by running an insert query there. Similarly, I can also load the IDs of the blocked countries from the database, but how can I tick the respective select boxes in the list of countries?
My initial "quick and dirty" idea was to use jQuery selectors inside a document.ready(), but the code gets inserted before everything else and, as such, it won't work because jQuery isn't even loaded yet.
How can this be done?
Cheers

Answer 1

I solved it by using the following code right before the end of the renderForm() function.
The Pièce de résistance was $this->fields_value, as sadly I didn't known of its existence.
public function getRestrictedCountries($obj)
{
// Loading blacklisted countries
$country = Db::getInstance(_PS_USE_SQL_SLAVE_)->executeS('
SELECT DISTINCT id_country
FROM `'._DB_PREFIX_.'category_country_restriction`
WHERE id_category = ' . (int)Tools::getValue('id_category') . ';');
$blacklisted_countries = array();
if (is_array($country))
foreach ($country as $cnt)
$blacklisted_countries[] = $cnt['id_country'];
// Global country list
$c_todos = Country::getCountries(Context::getContext()->language->id);
// Crossmatching everything
foreach ($c_todos as $c)
$this->fields_value['restricted_countries_'.$c['id_country']] = Tools::getValue('restricted_countries_'.$c['id_country'], (in_array($c['id_country'], $blacklisted_countries)));
}
PS: The table I am reading from is basically an associative table between 'category' and 'country'