Custom permalink with child taxonomy terms

This may ressemble other questions, but i’m posting my own as I couldn’t find a conclusive answer that worked for me.

What you need to know

I have a custom post type (products), with a custom taxonomy (collections). My collections taxonomy is hierarchical. For example, I have a parent collection (bathroom) which will have a good number of sub collections.

What i’m trying to do

I’m trying to obtain permalinks for my products which include both the parent collection and the child collection, example

 http://<domain>/products/bathroom/collection-1/postname

What i’ve tried

Basically i’ve been playing around with the rewrite parameter when registering my CPT and Taxonomy, as well as the ‘post_type_link’ hook.

I’ve managed to get it working partially, that is, by displaying only the parent collection in the permalink.

Any help would be greatly appreciated, thank you.

Also, my permalink structure is : /%category%/%postname%/

Some of my code

Registering CPT / Taxonomy

 register_taxonomy( 'collection', array(  ), array(
      'label' => 'collections',
      'public' => TRUE,
      'show_ui' => TRUE,
      'hierarchical' => TRUE,
      'query_var' => 'collections',
      'rewrite' => TRUE
 ));

 register_post_type( 'product', array(
      'label' => 'products',
      'public' => TRUE,
      'publicly_queryable' => TRUE,
      'show_ui' => TRUE,
      'show_in_menu' => TRUE,
      'taxonomies' => array( 'collection' ),
      'supports' => array( 'title', 'editor', 'author', 'custom-fields' ),
      'rewrite' => array( 'slug' => 'products/%collection%', 'with_front' => false, 'hierarchical' => true )
 ));

flush_rewrite_rules();

post type link hook

 if( strpos( $permalink, '%collection%' ) === FALSE )
      return $permalink;

 $terms = wp_get_object_terms( $post->ID, 'collection' );
 $tax_slug = "";

 if( empty( $terms[0]->parent ) )
 {
      $tax_slug = $terms[0]->slug;// . "/" . $terms[1]-> slug;
      //Second part 404's my permalinks, so commented out
 }
 else
 {
      $tax_slug = $terms[1]->slug;// . "/" . $terms[0]-> slug;
      //Second part 404's my permalinks, so commented out
 }

 $permalink = str_replace( '%collection%', $tax_slug, $permalink );

 return $permalink;