Checking for custom field value inside function.php

function child_exclude_category($query) {

global $post;
$cat = get_post_meta($post->ID, 'artikel', true);
if ($cat =='') { $cat = '-1,-3,-12'; }
else { $cat = '-1,-3'; }

	if ( $query->is_home ) {
		$query->set('cat', $cat);
		$query->set('orderby', 'date');
		$query->set('order', 'desc');
		$query->set('posts_per_page', '20');
	}
return $query;
}

add_filter('pre_get_posts', 'child_exclude_category');

I have a question regarding checking for a customfied inside function.php for the childtheme.

As you can see in the example above – i would like to check if a certain custom field value exists and if not a certain categori 12 should not be included in the query_post.

But i dosnt seem to work 🙁
What am i doing wrong

global $post;
$cat = get_post_meta($post->ID, 'artikel', true);
if ($cat =='') { $cat = '-1,-3,-12'; }
else { $cat = '-1,-3'; }