Select Advanced: Populate Options from Custom Model based on another field value

Hello,

If you use the custom model, it does not work with MB Relationships or post field. So you will need to create a custom callback function for the select_advanced field to display a list of models and it is beyond our scope of support.
If you are not able to do that, please contact us here https://metabox.io/contact/
and submit a customization service request, our development team will help you with an extra fee.

Hello,

>> How to get box fields’ values inside the callback function?
To get a normal field value inside the callback function, you can use the function get_post_meta(). To get a field value of a model, you need to use an SQL query.

>> What is the filter/action for setting the field options?
I do not understand this question. If you want to customize the options for a field, you can use the filter hook rwmb_normalize_{$field_id}_field. Refer to the documentation https://docs.metabox.io/filters/rwmb-normalize-field/

Hello,

You can try to use this code to get the post ID in the admin area

$post_id = null;
if ( isset( $_GET['post'] ) ) {
    $post_id = intval( $_GET['post'] );
} elseif ( isset( $_POST['post_ID'] ) ) {
    $post_id = intval( $_POST['post_ID'] );
}