Extending the search function to a specific meta field value in the admin posts table

Question

I'd like to transfor the following code. This code was done to add a specific search for post ID numbers. Now I need a function to extend the search to a meta field called "function_name", and the search is not public but from the admin post table

function search_by_post_id( $query ) {
    if ( $query->is_search ) {
        if ( is_numeric( $query->query_vars['s'] ) ) {
            $query->set( 'post_type', 'any' );
            $query->set( 'post__in', array( (int) $query->query_vars['s'] ) );
            $query->set( 's', '' );
        }
    }

    return $query;
}
add_filter( 'pre_get_posts', 'search_by_post_id' );

Answer 1

You can use the meta_query parameter of WP_Query to search custom fields.

Untested:

$meta_query = $query->get( 'meta_query', array() );

$meta_query[] = array(
    'key'     => 'function_name',
    'value'   => $query->query_vars['s'],
    'compare' => 'LIKE',
);

$query->set( 'meta_query', $meta_query );