在Symfony，MongoDB中查找ArrayCollection中元素的平均值

use Doctrine\Common\Collections\ArrayCollection;

class Item
{
    private $values;

    public function __construct()
    {
        $this->values = new ArrayCollection();
    }

    public function getAverageValue()
    {
        $sum = $this->values->reduce(function($carry, $item) {
            return $carry + $item;
        });

        $count = $this->values->count();

        if ($count > 0) {
            return $sum / $count;
        }

        return 0;
    }
}

class ItemController extends AbstractController
{
    public function averageAction(Item $item)
    {
        $averageValue = $item->getAverageValue();

        // 可以将平均值传递给视图进行展示
        return $this->render('item/average.html.twig', ['averageValue' => $averageValue]);
    }
}

db.items.aggregate([
  {
    $unwind: "$values"
  },
  {
    $group: {
      _id: null,
      averageValue: { $avg: "$values" }
    }
  }
])