Home Forums General Issues Setting the post Featured Image based on (PDF) File upload field thumbnail


Setting the post Featured Image based on (PDF) File upload field thumbnail

  • I’m trying to set the post Featured Image to be the thumbnail of a PDF that is uploaded to a file field (if the post has the category ‘publication’).

    The following code I wrote works when I first upload a file to the field, but once I remove that file and replace it with another, I get a 503 error when I save the post again/when I load Posts in the backend/load any page that loads that post. I then have to use wp_trash_post to delete that post to stop it wreaking havoc on my WordPress installation.

    function set_publication_post_thumbnail($post_id) {
    	if(has_category('publication')) {
    		$publication = get_field('publication', $post_id);
    		$attachment_id = $publication['ID'];
    		update_post_meta($post_id, '_thumbnail_id', $attachment_id);
    add_action('save_post', 'set_publication_post_thumbnail');

    Appreciate your help with this

  • This code is actually working – it’s some other code I have written that causes the 503 error!

  • Hi Damiane,

    I do need exactly the same, great help. But is your code still working with WP 5.9.2?

    I tried it, but nothing happens.

    Best regards


  • Hey mate,

    Yes, the code is still working with 5.9.2.

    A couple things that you need to make sure of –

    • My code has a condition checking if the post category is “Publication”, remove that if you don’t need it or update it according to the category you may need to check for
    • I am using the field type “File”, with the return value of File Array (not File URL or File ID)

    Let me know if you still can’t get it working after checking the above.

  • Hi Damiane,

    i already did exchange every “publication” with “downloadcenter”, wich is the name of my category, but had file URL as type. Now i changed it to “File Array”, but still nothing happens:
    I open a post with category “downloadcenter”, delete the attached file and assign a new file (PDF) and publish it. Now there should be a featured image, I guess – but there isn’t.

    Here’s my code:

    function set_downloadcenter_post_thumbnail($post_id) {
    	if(has_category('downloadcenter')) {
    		$downloadcenter = get_field('downloadcenter', $post_id);
    		$attachment_id = $downloadcenter['ID'];
    		update_post_meta($post_id, '_thumbnail_id', $downloadcenter_id);
    add_action('save_post', 'set_downloadcenter_post_thumbnail');
  • This reply has been marked as private.
  • Hi Damiane, I can’t see your reply as it has been marked as private.

  • Whoops sorry @ulinn

    Can you check your Site Health on the WordPress dashboard and check that you have all the PHP extensions that WordPress needs? I’ve previously had an issue where WordPress could not create a thumbnail for a PDF because there was a missing extension.

    Otherwise I’d be happy to take a quick look at the site if you want me to.

  • Site health is good, PHP version is 7.4.5

    If I post credentials and mark them as “private”, only you will see them, correct?

  • Unfortunately I don’t think that is how the Private visibility works – that’s what I tried to do with my earlier reply. So don’t put your credentials on here!

  • Is the slug you’ve used inside has_category() correct?

  • The ACF field Label is “Download”, and the field Name is “downloadcenter”.
    The category name is “Downloadcenter” and the category slug is “downloadcenter”.

  • An FYI RE Private Comments. These can be seen only by the thread starter and someone that has admin access on the forum. A long time ago the original author of ACF used these forums for support. This is no longer the case but the feature still lingers.

  • @hube2 Thank you for making clear, good to know.

  • Hey @ulinn

    Sorry I just took another look at your code and found the issue – in update_post_meta, you are setting the value of the _thumbnail_id meta to the variable $downloadcenter_id which has not been defined.

    Update that line to the following –

    update_post_meta($post_id, '_thumbnail_id', $attachment_id);

    Test and let me know if you still have any issues with it!

  • Hi @damianemuller,

    thank you for looking over my code. I could have realized this myself, sorry.

    But: Also with the updated line nothing happens. I did change the attachment of a post, updated the post – but there still is no featured image.

  • I found part of the problem – my child theme was not active, so the code did not work. Sorry for that …

    After activating there is still no thumbnail, but now there is a “replace featured image” button in my post (see screenshot), so something happend to the post. But it does not to seem a vaild image file.

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