在laravel mongodb中对数组对象求和

Question

I am working on mongoDB with Laravel using jenssegers. And i am trying to sum based on Abonos.price. I have a MongoDb document like this:

`{ 
"_id" : ObjectId("594a89358b85112444002924"), 
"cliente" : "blabla", 
"tipo" : "bla bla", 
"paquete" : "bla bla", 
"date" : "2017-06-14", 
"content" : "picture", 
"cost" : NumberInt(200), 
"status" : NumberInt(2), 
"Abonos" : [
    {
        "price" : "200", `enter code here`
        "date" : "2017-06-21"
    }, 
    {
        "price" : "300", 
        "date" : null
    }
], }`

I want to sum Abonos.price to get "500" (200+300) I have this function in my laravel controller:

$result = Work::raw(function($collection){
    return $collection->aggregate(array(
       array('$unwind' => '$Abonos'),
       array('$group' => array(
           "_id" => '$_id',
            "total" => array('$sum' => '$Abonos.price')
       )),
  ));});
return $result;

But i get

[{"_id":"594a95cc8b85112444002925","total":0}

instead of

[{"_id":"594a95cc8b85112444002925","total":500}"

I would like to make $return = 500