Laravel MongoDB自我参考

I made a Laravel 5 application with MongoDB. For MongoDB, I'm using jenssegers/laravel-mongodb.

I have a MongoDB object like this:

{
    "_id" : ObjectId("556c47677347b041b6004bfd"),
    "name" : "Lucht?",
    "type" : "onderzoeksvragen",
    "levels" : [ 
        "5", 
        "6"
    ],
    "data" : {
        "name" : "Lucht?",
        "description" : "<p>
\tWat is lucht?</p>
<p>
\tHoe sterk is lucht?</p>
<p>
\tWat kun je met lucht?</p>
",
        "coach_information" : null
    },
    "updated_at" : ISODate("2015-06-01T11:52:23.399Z"),
    "created_at" : ISODate("2015-06-01T11:52:07.536Z"),
    "related_ids" : [ 
        "556c47627347b041b6004263"
    ]
}

The field related_ids contains an array with ObjectId's who are related to this object. I want to get this object with the related objects. The related objects are the same type as the main object. So, they are in the same collection.

I have read the documentation on Github (https://github.com/jenssegers/laravel-mongodb) and found this:

The belongsToMany relation will not use a pivot "table", but will push id's to a related_ids attribute instead.

use Jenssegers\Mongodb\Model as Eloquent;

class User extends Eloquent {

    public function groups()
    {
        return $this->belongsToMany('Group', null, 'users', 'groups');
    }

}

But, it doesn't work for me. My code:

Controller ExploreController.php

$explore = Explore::with('related')->find('556c47677347b041b6004bfd');

Model Explore.php

public function related()
{
    $this->belongsToMany('App\Explore', null, 'explore' 'explore');
}

The result of this includes only the main object, not the related objects.