validate before add to cart not working woocommerce - wordpress

Question

i have something i cant get working and i dont understand why.
I want to add some custom fields below a grouped product in woocommerce on the frontend.
So i have the following function (which works fine):
function add_data_opbouw_front(){
global $woocommerce; global $post;
$id = $post->ID;
if ($id === 1411){
$optiestijd = array();
$optiesdag = array();
$id = get_the_ID();
$html = '<table><tbody>';
$i=0;
if( !empty(get_post_meta($id,'tijdslot',true) ) ){
$html .= '<tr><td>Kies een tijd:</td><td><select name="tijdslot" required><option value="">Kies:</option>';
$optiestijden = get_post_meta(get_the_ID(),'tijdslot',true);
foreach ($optiestijden as $key=>$optie){
$optiestijd[$optie] = 'Kies een tijdslot:';
}
foreach ($optiestijd as $optie => $label){
if(!empty($optie)){
$i++;
$html .= '<option value="'.$optie.'"><span class="choiceoption">'. $optie.'</span></option>';
}
}
$html .= '</select></td></tr>';
}
if( !empty(get_post_meta($id,'dag',true) ) ){
$optiesdagen = get_post_meta(get_the_ID(),'dag',true);
$optiesdagen = explode('|',$optiesdagen);
$html.= '<tr><td>Kies een dag:</td><td><select name="dag" required><option value="">Kies:</option>';
foreach ($optiesdagen as $optie){
$i++;
$html .= '<option value="'.$optie.'"><span class="choiceoption">'. $optie.'</span></option>';
}
}
$html .= '</select></td></tr></tbody></table>';
$html .= '<input type="hidden" name="voorraad_afname" value="1">';
echo $html;
}
}
add_action('woocommerce_before_add_to_cart_button','add_data_opbouw_front', 35);
because i want to obligate user to pick a day and time, i want to validate before the add to cart and i do it like this:
function so_validate_add_cart_item( $passed, $product_id, $quantity, $variation_id = '', $variations= '' ) {
global $woocommerce; global $post;
$id = $post->ID;
if ($id === 1411){
// do your validation, if not met switch $passed to false
if ( empty($_POST['tijdslot']) || empty($_POST['dag']) ){
$passed = false;
wc_add_notice( __( 'De velden "Kies een dag" en "Kies een tijdslot" zijn beiden verplicht', 'textdomain' ), 'error' );
}
}
return $passed;
}
add_filter( 'woocommerce_add_to_cart_validation', 'so_validate_add_cart_item', 10, 5 );
But when I leave one of the select boxes empty, the product is being added to the cart anyway. So the validate doesnt throw in the error. I dont really understand why this is not working.
Any ideas?

Answer 1

After a long puzzle, i found out that approaching global $post is not working. Better is to approach the product_id. Because it is a grouped product, i checked wether one of the grouped products ID are in the add-to-cart like so:
$check = array('1398','1399','1400');
if (in_array($product_id,$check)){
And that does work.
So updated code is:
function so_validate_add_cart_item( $passed, $product_id, $quantity, $variation_id = '', $variations= '' ) {
$check = array('1398','1399','1400');
if (in_array($product_id,$check)){
// check if both selectboxes are used
if ( empty($_POST['tijdslot']) || empty($_POST['dag']) ){
$passed = false;
wc_add_notice( __( 'De velden "Kies een dag" en "Kies een tijdslot" zijn beiden verplicht', 'textdomain' ), 'error' );
}
}
return $passed;
}
add_filter( 'woocommerce_add_to_cart_validation', 'so_validate_add_cart_item', 10, 5 );