belongsToMany not working in nested collection

[callumthomson]

Short: when using the belongsToMany relationship between a top level document, and a document that is nested in another document, the related_ids property is not set properly on the nested document.

I have three models, App\User, App\User\WishList, and App\Product. App\User and App\Product represent base collections, but App\WishList exists only inside User.

App\User model:

namespace App;

use Jenssegers\Mongodb\Auth\User as Authenticatable;

class User extends Authenticatable
{

    protected $collection = 'users';
    protected $guarded = [];

    public function wish_lists()
    {
        return $this->embedsMany('App\User\WishList');
    }
}

App\User\WishList model:

namespace App\User;

use Jenssegers\Mongodb\Eloquent\Model;

class WishList extends Model
{
    protected $guarded = [];

    public function products()
    {
        return $this->belongsToMany('App\Product');
    }
}

App\Product model:

namespace App;

use Jenssegers\Mongodb\Eloquent\Model;

class Product extends Model
{
    protected $collection = 'products';
    protected $guarded = [];

    public function wish_lists()
    {
        return $this->belongsToMany('App\User\WishList');
    }
}

Test: I want to add a product to a users wishlist.

$user = User::first();
$product = Product::first();
$wishList = $user->wish_lists->first();

$wishList->products()->attach($product->id);

Expected:

• wish_list_ids array property added to the document in products collection, containing the ID of the users wish list.
• product_ids array property added to user.wish_lists[0] in users collection, containing the ID of the product.
• Retrieve products in wish list by calling $wishList->products
• Retrieve wish lists containing products by calling $product->wish_lists

Actual:

• wish_list_ids array property added to the document in products collection, containing the id of the users wish list.
• product_ids array property not added to document.wish_lists[0] in users collection.
• Retrieve products in wish list by calling $wishList->products
• Empty collection when calling $product->wish_lists

Example documents after running the code:

users collection:

{  
   "_id":"5d5704d8fdce27086910aec3",
   "name":[  
      "John",
      "Smith"
   ],
   "wish_lists":[  
      {  
         "name":"TEST_LIST",
         "_id":"5d59e736fdce271cf23b3a34"
      }
   ]
}

products collection:

{
   "_id":"5d55b24ae924eb31649d20b7",
   "name":"Product 1",
   "wish_list_ids":[
      "5d59e736fdce271cf23b3a34"
   ]
}

NOTE: I tried running this code, but using a base collection to store the wish lists, instead of storing them inside users, and it worked completely fine.