WPDB query suddenly not working

Question

I have a function that contains a wpdb query. It's supposed to get the image alt text, based on the url of the image. This is the function:

// retrieves the attachment alt from the file URL
function get_image_alt_from_url($image_url) {
  global $wpdb;
  $id = $wpdb->get_col($wpdb->prepare("SELECT ID FROM $wpdb->posts WHERE guid='%s';", $image_url ));
  if ($id) {
    return get_post_meta($id[0], '_wp_attachment_image_alt', TRUE);
  }
  return false;
}

It used to work, but now it returns false every time. Upon debugging the function, I found that the $id variable is null.

I wonder if this has anything to do with upgrading PHP to 8.0, and if so, how to get around it. It still works on my local site, so I doubt it's related to a wp core update.

WordPress: 6.2.2 PHP: 8.0 Hosted on Kinsta

Thank you so much for your help,

Jill

Answer 1

You should only interact with the DB via WP, not directly. It's not a big deal in your case because you just want to see a value, but you'll be more aligned with the best practices if you go in trying to solve a problem with $wpdb as a last resort. In your case, you can try this :

$id = attachment_url_to_postid($image_url);
$alt = get_post_meta($id, '_wp_attachment_image_alt', TRUE);

We'll first get the Post ID from the attachment URL, and then find the alt from that ID. This will do what you want without $wpdb.

That said, let's try and debug your original problem too!

As @PatJ said, WPDB::get_col() never returns null, so something's wrong. Turn on your debug like this :

define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );
define( 'WP_DEBUG_DISPLAY', false );

The first line enables debugging mode, the second line ensures that debug messages are logged to a wp-content/debug.log file, and the third line prevents debug messages from being displayed on the site.

Run it again. Was an error logged? If not, run this :

echo $wpdb->last_error;

Also, after your get_col line, print the last query using the $wpdb->last_query command. This can help you make sure the query is as expected :

echo $wpdb->last_query;

Of course, check the image URL too - maybe it's not correct, or it's not in the database. It shouldn't return null, but worth checking.

Also, it's possible that the issue is not with your $wpdb->get_col() line but the get_post_meta() line. You can debug this by checking what it returns when you give it a known post ID and '_wp_attachment_image_alt' as the meta key.