form validation in_list

Yep, but you need to implode it first. So that your array turns into comma separation instead.

PHP Code:

$array = [0 => 'red', 1 => 'green', 2 => 'blue'];
$array = implode($array,',');
// Array now looks like:
$array = 'red,green,blue';

// Use it like this
// I'm using array_keys() here because I want 0,1,2 instead of red,green,blue.
array(
    'field' => 'menus_department[]',
    'label' => lang('groups_type_department'),
    'rules' => array(
        'trim',
        'required',
        'in_list['.implode(array_keys($this->data['groups']['department']),',').']'
    )
),