display custom-field-defined thumbnail image as featured image in blog listing?

Hi there,

if you're using the Block Editor then you can use the Block Element and GenerateBlocks to construct your own Content Template for your blog/archives - see here:

https://docs.generatepress.com/article/block-element-content-template/

The examples provided use the GP Dynamic Image block, which can be set to Featured Image - but in your case you would use the Post Meta option and enter your CF name.

This method will also allow you to create that custom layout.

Alternatively Tom provides a method here for swapping out the featured image for the custom field:

https://generatepress.com/forums/topic/cant-cover-generate_post_image-via-function-php/#post-1738181

Well, I tried this, but it didn't work:

add_filter( 'has_post_thumbnail', function( $has, $post ) {
    $custom_field_image_url = get_post_meta( $post, $key = 'Thumbnail', TRUE);

    if ( $custom_field_image_url ) {
        $has = true;
    }

    return $has;
}, 10, 2 );

add_filter( 'generate_featured_image_output', function( $output ) {
    if ( is_home() ) {
        if ( has_post_thumbnail() ) {
            return sprintf( // WPCS: XSS ok.
                '<div class="post-image">
                    <a href="%1$s">
                        %2$s
                    </a>
                </div>',
                esc_url( get_permalink() ),
                get_the_post_thumbnail(
                    get_the_ID(),
                    apply_filters( 'generate_page_header_default_size', '' ),
                    array('itemprop' => 'image' )
                )
            );
        }
    }

    return $output;
} );

(I am very far from an expert in PHP.)

This is how I've got the recent-posts page customized re featured images:

I'd like to give the GenerateBlocks method a try, but I'm having a bizarre technical issue with that. Will start another topic for that.

Thanks.

Lets deal with this code first:

add_filter( 'has_post_thumbnail', function( $has, $post ) {
    $custom_field_image_url = get_post_meta( $post, $key = 'Thumbnail', TRUE);

    if ( $custom_field_image_url ) {
        $has = true;
    }

    return $has;
}, 10, 2 );

This line:

$custom_field_image_url = get_post_meta( $post, $key = 'Thumbnail', TRUE);

needs changing to:

$custom_field_image_url = get_post_meta( $post, 'YOUR_CUSTOM_FIELD_NAME', TRUE);

Change the YOUR_CUSTOM_FIELD_NAME to the name (key) for your CF

Okay, done. (I don't see any difference, but I suspect there's more to come.)

In Customizer > layout > blog --> Featured Images is the Display featured images checked for Archives?

Yes, the Customizer window is set up as in the screengrab above.

Try this snippet instead:

function db_custom_field_featured_image( $html, $post_id ) {
    $custom_url = get_post_meta( $post_id, 'YOUR_CUSTOM_FIELD_NAME', true );
    if ( '' != $custom_url ) {
        return '<img src="' . $custom_url . '" />';
    } 
    else {
        return $html;
    }
}
add_filter( 'post_thumbnail_html', 'db_custom_field_featured_image', 10, 2 );

I'm afraid that doesn't work either.

Are you sure the custom field name is correct ?

I think so. Here's what it looks like on a post page:

And here's the totality of the snippet (I'm using the Code Snippets plugin):

function db_custom_field_featured_image( $html, $post_id ) {
    $custom_url = get_post_meta( $post_id, 'Thumbnail', true );
    if ( '' != $custom_url ) {
        return '<img src="' . $custom_url . '" />';
    } 
    else {
        return $html;
    }
}
add_filter( 'post_thumbnail_html', 'db_custom_field_featured_image', 10, 2 );

Is there something I'm missing, or doing wrong?

I am no expert in any of this and do appreciate your handholding here!

Hmmm... can you try making it lowercase so in the PHP Snippet Thumbnail becomes thumbnail

Done. It didn't help.

Ok - so it looks like the post isn't support the post thumbnail so it doesn't render the HTML for it.
So lets give this a shot instead:

add_action('generate_before_entry_title', function(){
    $custom_url = get_post_meta( get_the_ID(), 'thumbnail', true );
    if (!is_single() && !empty($custom_url)) {
        echo '<img src="' . $custom_url . '" />';
    }
});

Progress! That gets the thumbnail on the page (with "Thumbnail" capitalized), but the customizer no longer works to style the thumbnail. I achieved that with CSS:

.blog .entry-header img, .archive .entry-header img {
  max-width: 250px;
  height: auto;
  float: left;
  margin-right: 15px;
}

That seems to work. Do you see any issues with this?