How to check if an article is referenced from a collection?
To load all collection id's, which have a reference to $article_id:

$collection_ids = \Drupal::entityQuery('node')
  -&gt;condition('type', 'collection')
  -&gt;condition('field_article_ref', $article_id)
  -&gt;execute();
if ( count($collection_ids) &gt; 0 ) {
  echo "the article $article_id is referenced in one or more collections";
}