Is there any action filter/hook for validating a custom field before publishing the post?

Question

I have a custom filed called xxxx_url. xxxx_url should be unique.

So, before publishing the post, I want to make sure that xxxx_url is unique or not? If it isn't unique, publishing the post should be reject.

I tried publish_post. But that isn't the correct one, since it triggers when we publish the post. I want to run my code just before the publish.

Answer 1

At the beginning of wp_insert_post, the function that saves/updates a post, there is a filter called wp_insert_post_empty_content. By default this filter checks whether the title, editor, and excerpt fields are all empty, in which case the save process will be halted.

However, since all the fields to be saved are passed to this filter, you can expand this filter to include any other test to determine whether the post should be considered empty. It would be something like this:

add_filter ('wp_insert_post_empty_content','wpse312975_check_unique_url',10,2);
function wpse312975_check_unique_url ($maybe_empty, $postarr) {

  // extract custom field from $postarr, check uniqueness

  if ($unique) return false else return true;
  }

Note: the function must return 'true' to halt the saving process.

If the custom field is not unique you may also want to echo a warning.

Answer 2

I would hook into wp_insert_post_data filter and handle this as less intrusive as possible because as I understand you don't want to block the insertion of a post, you just want to avoid publishing a post with duplicate meta values.

I cannot spare too much in this case because you didn't share any code but here is some pseudo-code of a filter that would work:

function wp8193131_check_if_meta_value_is_unique ( $data, $postarr ) {
    // setup an uniqueness flag.
    $meta_is_unique = true;

    // check if the meta is unique and modify the `$meta_is_unique` flag accordingly.
    // {...} <- your code

    // if the meta is NOT unique keep the post status in draft.
    if ( ! $meta_is_unique ) {
        // you can force the current post to be draft until the meta value will became unique.
        $data['post_status'] = 'draft';
        // maybe, update the meta value with a hint of the fact that it's not unique.
        // or display a dashboard notice about it.
    }

    return $data;
}
add_filter( 'wp_insert_post_data', 'wp8193131_check_if_meta_value_is_unique' );

Another great thing about this filter is that it's separated from the attachments one wp_insert_attachment_data.

I hope it helps, and whatever you do, it sounds awesome!

Answer 3

How about using AJAX to check the uniqueness before even submitting the post for publishing?

$( '#post' ).on( 'submit', function( event ) {
  event.preventDefault(); // Prevent publishing

  //Now do some AJAX Checks
  $.post( ajaxurl, data, function(response) {
    if ( response === 'success' ) {
        $( this ).off( event ).submit();
    } else {
        alert( 'The custom field must be unique' );
    }
  });
});  

Though the code isn't tested, but it should work. You might need to play with it to get your desired result.

Answer 4

The check should go to wp_insert_post. This hook is fired whenever post is published or edited.

There, you can do a custom query to check, if there is any post already has the same xxxx_url value or not.

add_action('wp_insert_post', function($post_id) {
    $meta_key = 'xxxx_url';
    $meta_value = get_post_meta($post_id, $meta_key, true);
    $query = new WP_Query([
        'post_type' => get_post_type($post_id),     // This might be unnecessary, if you check `post` post type only. Or use `any`.
        'meta_query' => [
            [
                'meta_key' => $meta_key,
                'meta_value' => $meta_value,
            ]
        ]
    ]);
    if ($query->have_posts()) {
        // invalid key, post with the same value already exists
    } else {
        // valid, key was not found anywhere
    }
});