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  • Thanks John! I have no idea what I’m doing but your code got me close. I’m making a lyrics site / hymn book and want to prevent duplicate hymns, but I’m stuck. Can you get my code to validate?

    add_filter('acf/load_value/name=hidden_post_id', 'set_hidden_post_id_value', 10, 3);
    function set_hidden_post_id_value($value, $post_id, $field) {
      // set the value of the field to the current post id
      return $post_id;
    add_action('admin_head', 'hide_hidden_post_id');
    function hide_hidden_post_id() {
        <style type="text/css">
          /* the field key for the post id field */
          div[data-field_key="field_581bff482f898"] {
            display: none;
    add_filter('acf/validate_value/type=number', 'require_unique', 10, 4);
    function require_unique($valid, $value, $field, $input) {
      if (!$valid) {
        return $valid;
      // get the post id
      // using field key of post id field
      $post_id = $_POST['acf']['field_581bff482f898'];
      // query existing posts for matching value
      $args = array(
        'post_type' => 'hymn',
        'posts_per_page' => 1, // only need to see if there is 1
        'post_status' => 'publish, draft, trash',
        // don't include the current post
        'post__not_in' => array($post_id),
        'meta_query' => array(
            'key' => $field['type'],
            'value' => $value
      $query = new WP_Query($args);
      if (count($query->posts)){
        $valid = 'This Value is not Unique';
      return $valid;

    I added the hidden_post_id to the field group in the ACF menu.The hidden_post_id is working, although the acf/load_field wasn’t working for some reason.

    acf/load_field gives me issues similar to these:

    Lastly, while WP_Query looks like a better solution. Is there an easier way to do it with $wpdb?