Facebook social publisher and custom post type fields [closed]

Question

I have a problem with how Facebook plugin shows my WordPress-based posts on my timeline. What basically happens is that the post image isn't shown.

Now, the post image I want to show is a custom field of a custom post type, and this is probably what's causing the problem to the Facebook plugin.

On the single-post page I added manually the og: tags, and they're working properly (I see the image if I share the link on Facebook, and the Facebook object debugger shows it too, saying there are no problems).

Do have I to do something to add manually the requested og: tags in post_publish hook?

A sample page is this: http://www.zannoni-store.it/prodotti/canestro-sonoro

And what I added in order to let the image be shown on share is this:

..
$img = get_post(get_post_meta($post->ID, 'immagine', true)); 
echo '<meta property="og:image" content="'.$img->guid.'" />';
..

Answer 1

reading the code of the Facebook Official Plugin I've seen that in

open-graph-protocol.php

It's being applied the custom filter 'fb_meta_tags':

..
$meta_tags = apply_filters( 'fb_meta_tags', $meta_tags, $post );
..

So in my plugin I've added a filter for 'fb_meta_tags' that adds/replace the image set from the Facebook plugin in this way:

function clancat_product_fbmeta($meta_tags, $post){
    ..
    $img = get_post(get_post_meta($post->ID, 'immagine', true)); 
    $meta_tags['http://ogp.me/ns#image'] = $img->guid;
    ..
    return $meta_tags;
}

..

add_filter( 'fb_meta_tags', 'clancat_product_fbmeta',10,2);

Please note that the "2" (4th param passed to add_filter) is mandatory, because the Facebook filter it's providing the $meta_tags array and the $post object.

I think this solution ca be easily extended to any post-type / custom-field etc. situation; and that's the correct way to achieve this goal.

Unfortunately this not solve the publish problem.