从聚合中重新格式化MongoDB结果

Question

我来自SQL世界，从MongoDB开始，我仍然有点困惑。。。我有一个这样的收藏

{
    "_id" : ObjectId("5769b51f675e6190119935ec"),
    "city" : "City1",
    "company" : "Company1",
    "country" : "Country1",
    "zip" : "23345",
},{
    "_id" : ObjectId("5769b51f675e6190119935ed"),
    "city" : "City1",
    "company" : "Company2",
    "country" : "Country1",
    "zip" : "1245",
},{
    "_id" : ObjectId("5769b51f675e6190119935ee"),
    "city" : "City2",
    "company" : "Company1",
    "country" : "Country1",
    "zip" : "123445",
},{
    "_id" : ObjectId("5769b51f675e6190119935ef"),
    "city" : "City1",
    "company" : "Company2",
    "country" : "Country1",
    "zip" : "1235445",
}

还有我的问题，

db.getCollection('stores').aggregate([{"$group":{"_id" : {city :"$city", company : "$company"}}}])

我正在使用角和NodejS Express从数据库中获取数据，我以这种格式获取数据

[
    {
        _id:{
            city:"City1",
            company:"Compnay1"
        }
    },
    {
        _id:{
            city:"City1",
            company:"Company2"
        }
    },
    {
        _id:{
            city:"City2",
            company:"Compan1"
        }
    }
]

因此，我想知道是否有一种方法可以在没有id键的情况下获得此查询结果，

像这样：

 [
        {
            city:"City1",
            company:"Compnay1"
        },
        {
            city:"City1",
            company:"Company2"
        },
        {
            city:"City2",
            company:"Compan1"
        }
 ]

Answer 1

只需在下一个管道阶段使用$project作为聚合管道运算符：

db.getCollection('stores').aggregate([
  {"$group":{"_id" : {city :"$city", company : "$company"}}},
  {"$project": {"city": "$_id.city", "company": "$_id.company", "_id": 0}}
])