Problem: I have a Form that is used to create child post in a one-to-many post relationship. I would like to get the value of a custom field from the related parent post in a cred_save_data callback, but it does not seem to be working.
Solution: The relationship is not yet set in a cred_save_data hook, so you can either use the cred_submit_complete hook instead, or access the parent post ID directly from the parent select field in the $_POST superglobal.
$parent_post_id = $_POST['@relationshipslug_parent'];
$parent_field_value = get_post_meta( $parent_post_id, 'wpcf-fieldslug', true);