如何在MONGODB中加入四个集合

Question

我需要您的帮助。 我有4个收藏集.. 国家：

{Country:[
     {
        "MENU": "CNTY",
        "DESP": "India",
        "CODE": "IND"
    }]
}

状态：

{
    State: [
        {
            "MENU": "STATE",
            "CNTY": "IND",
            "DESP": "Andhra Pradesh",
            "CODE": "AP",
            "TINCODE": "37"
        }]
}

地区

{District:[
         {
    "CODE": "504",
    "DESP": "KADAPA",
    "STATE": "AP",
    "CNTY": "IND"
}
]}

小镇

{Town:[{
            "Menu": "TOWN",
            "CODE": "AP03546",
            "District": "504",
            "DESP": "Abbavaram",
            "Pincode": "516270"
        }]
}

我已经尝试使用汇总和查找

DBclient.db("Dictionary").collection("Country").aggregate([{
    $lookup: {
      from: "State",
      localField: "CNTY",
      foreignField: "CODE",
      as: "states"
    }
  }, {
    $unwind: "$states"
  },
  {
    $lookup: {
      from: "Districts",
      localField: "states.CODE",
      foreignField: "STATE",
      as: "district"
    }
  }, {
    $unwind: "$district"
  },
  {
    $lookup: {
      from: "Towns",
      localField: "district.CODE",
      foreignField: "District",
      as: "towns"
    }
  }, {
    $unwind: "$towns"
  }
]).toArray();

我得到了null数组。这个查询有什么问题..plz给我建议。