在Doctrine（ODM）中查询无效

Question

我在Doctrine和MongoDB（ODM）中遇到问题。这是我的文档示例：

 [ 
  {
    "id": "some_id",
    "country": "Germany",
    "cities": [{
        "city": "Munich"
    }, {
        "city": "Berlin"
    }, {
        "city": "Hamburg"
    }]
   },
{
    "id": "some_id",
    "country": "Russia",
    "cities": [{
        "city": "Moscow"
    }, {
        "city": "Saint Petersburg"
    }, {
        "city": "Samara"
    }]
   },

{
    "id": "some_id",
    "country": "United Kingdom",
    "cities": [{
        "city": "London"
    }, {
        "city": "Birmingham"
    }, {
        "city": "Bristol"
    }]
   }

]

我想返回一个来自俄罗斯城市的数组，这是我当前的查询：

$query = $this->createQueryBuilder()
            ->select('cities')
            ->field('country')->equals('Russia')
            ->hydrate(false)
            ->getQuery()
            ->execute();
        $result = iterator_to_array($query);

$result返回每个国家/地区所有城市的数组，但我只获得俄罗斯的城市。

谢谢！