https://www.advancedcustomfields.com/resources/acf-fields-relationship-query/
This could very likely time out the field and stop it from showing results.
The only way that I can thing to do this (well, the only way without querying the db directly) is to build a filter as suggested by @csaborio and in the filter get all other posts of the same time and look at what’s already been selected.
add_filter('acf/fields/relationship/query/name=related_tracks', 'remove_already_selected', 10, 3);
function remove_already_selected($args, $field, $post_id) {
$query = new WP_Query(array(
'post_type' => 'album', // my assumption
'post_status' => 'publish',
'posts_per_page' => -1
));
$selected = array();
if (count($query->posts)) {
foreach ($query->posts as $post) {
$tracks = get_field('related_tracks', $post->ID, false);
if (!empty($tracks)) {
foreach ($tracks as $track) {
$track = intval($track);
if ($track && !in_array($track, $selected) {
$selected[] = $track;
}
} // end foreach track
} // end if tracks
} // end foreach post
} // end if posts
if (!empty($selected)) {
$args['post__not_in'] = $selected ;
}
return $args;
} // end function
As I said, depending on the number of “Album” posts that exist this will eventually time out the AJAX request for the relationship field and would not be very scale-able.
You could potentially query the post meta table directly to get the acf field value for published posts.
Another possibility would be to create and acf/save_post. In this filter, when saving an “Album” post you could set an option (get_option()/update_option()) and store a list of selected tracks.
Yet another option would be to make the relationship bidirectional with a relationship on the Album side and a post object field on the track site. Then you could do something like this.
add_filter('acf/fields/relationship/query/name=related_tracks', 'remove_already_selected', 10, 3);
function remove_already_selected($args, $field, $post_id) {
$args['meta_query'] = array(
'relation' => 'OR'
array(
'key' => 'bidirectional_post_object_field_name',
'value' => ''
),
array(
'key' => 'bidirectional_post_object_field_name',
'compare' => 'NOT EXISTS'
)
);
return $args;
} // end function
