Problems with database query after upgrading

I have created a site, where the editor has a posibility to add related posts to a post.

Now, when a page is shown, all related posts are presented below the content. A related post is presented by a title, a thumbnail and some text. The text comes from a custom field (_custom_introduce_text) that I pick up using this:
$manchet = $wpdb->get_results("SELECT meta_value FROM wp_FreshFarm_postmeta WHERE post_id=".$key->ID." and meta_key='_custom_introduce_text'", 'OBJECT');

I will start by saying that this actually worked before upgrading to 3.5.1, so I’m at a loss why it’s not working anymore.

If I “hard code” the query to e.g.
$manchet = $wpdb->get_results("SELECT meta_value FROM wp_FreshFarm_postmeta WHERE post_id=476 and meta_key='_custom_introduce_text'", 'OBJECT');
then a result is returned.

If I write $key to the screen this is not empty.

But whenever $key is part of the query, nothing is returned from the database.

This is the “full” code that I’m processing:

<?php
$related_posts = MRP_get_related_posts( $post->ID );
    if( $related_posts ) {?><h2>Relaterede artikler</h2>
				<ul class="opsamlingPost">
<?php foreach( $related_posts as $key=>$value ) { ?>
						<li>
							<a href="<?php echo get_permalink($key); ?>">
								<?php echo get_the_post_thumbnail($key, array(220,220)); ?>
								<h2><?php echo get_the_title($key); ?></h2>
		<?php
		$manchet = $wpdb->get_results("SELECT meta_value FROM wp_FreshFarm_postmeta WHERE post_id=".$key->ID." and meta_key='_custom_introduce_text'", 'OBJECT'); 

		if( $manchet )
			foreach( $manchet as $my_manchet) { ?>
        <p><?php echo $my_manchet->meta_value; ?></p>
        <?php } ?>
							</a>
							</li>
<?php }
?>
				</ul>

Any help is much appreciated 🙂