Updated about 7 yrs, 8 mths ago (March 10, 2017). Know a better answer? Let me know!

Improve Advanced Custom Fields Relationship and Post Object Fields Search

How to get the ACF Relationship and Post Object fields to sort by search results relevance, only show published posts, search by URL, and exclude the current page

I often use the Advanced Custom Fields Relationship, Post Object or Page Link fields in WordPress, however I find that the search with these fields is lacking in these ways:

The search does not return results ordered by relevance, making it hard to find what you are looking for
The search can’t search by URL, making it difficult to find an exact post/page if there are many posts/pages being searched
The search will return the current page. This is usually not what you would want.
The search returns pages and posts that are drafts

Fortunately, Advanced Custom Fields has some filters we can use to modify this search behaviour.

Here is how to modify an ACF Relationship field to:

Only show published posts
Sort the results by relevance, making it much easier to find the post you are looking for
Search by a URL. To find an exact post, you can copy and paste the URL of that post into the search box
Exclude the current page from the results.

This code would normally go in your functions.php file, or possibly in a plugin.

function your_unique_function_name($args, $field, $post_id)
{
    // 1: only show published posts
    $args['post_status'] = array('publish');

    // 2: sort by relevance
    $args['orderby'] = 'relevance';
    $args['order'] = 'DESC';
    $args['posts_per_page'] = 500;

    // 3: search by URL if a URL is entered
    if(!empty($args['s']) && substr($args['s'], 0, 4) == 'http')
    {
        $post_id_to_search_for = url_to_postid($args['s']);
        if($post_id_to_search_for)
        {
            unset($args['s']);
            $args['p'] = $post_id_to_search_for;
        }
    }

    // 4: exclude current post/page
    $args['post__not_in'] = array($post_id);

    return $args;
}
add_filter('acf/fields/relationship/query/key=field_XXXXXXXXXXX', 'your_unique_function_name', 10, 3);

The field key needs to be the correct one, and there are different filters depending on whether you are using a Relationship or Post Object field. See the links below for further information.

Note that the field key is hidden by default. To show it:

Go to Screen Options at the top of the page
Select Field Keys

More Information

For more information:

Advanced Custom Fields
A WordPress plugin to add advanced custom fields to pages, posts, and elsewhere.
- Relationship Field
  - Relationship Field Query Filter
- Post Object Field
  - Post Object Field Query Filter

Updated about 7 yrs, 8 mths ago (March 10, 2017). Know a better answer? Let me know!