如何使用 MongoDB 聚合 `$lookup` 作为 `findOne()`

Question

众所周知，find() 返回一个结果数组，findOne() 只返回一个简单的对象。

对于 Angular，这会产生巨大的差异。我可以直接写{{myresult.name}}，而不是{{myresult[0].name}}。

我发现聚合管道中的$lookup 方法返回一个结果数组，而不仅仅是一个对象。

例如，我有两个集合：

users收藏：

[{
  "firstName": "John",
  "lastName": "Smith",
  "country": 123
}, {
  "firstName": "Luke",
  "lastName": "Jones",
  "country": 321
}]

countries收藏：

[{
  "name": "Australia",
  "code": "AU",
  "_id": 123
}, {
  "name": "New Zealand",
  "code": "NZ",
  "_id": 321
}]

我的聚合$lookup：

db.users.aggregate([{
  $project: {
    "fullName": {
      $concat: ["$firstName", " ", "$lastName"]
    },
    "country": "$country"
  }
}, {
  $lookup: {
    from: "countries",
    localField: "country",
    foreignField: "_id",
    as: "country"
  }
}])

查询结果：

[{
  "fullName": "John Smith",
  "country": [{
    "name": "Australia",
    "code": "AU",
    "_id": 123
  }]
}, {
 "fullName": "Luke Jones",
 "country": [{
   "name": "New Zealand",
   "code": "NZ",
   "_id": 321
 }]
}]

从上面的结果可以看出，每个country 都是一个数组，而不是像"country": {....} 这样的单个对象。

我怎样才能让我的 $lookup 返回单个对象而不是数组，因为它只会匹配单个文档？

Answer 1

db.users.aggregate([
    {
        $lookup: {
            from: 'countries',
            localField: 'country',
            foreignField: '_id',
            as: 'country'
        }
    },
    {
        $unwind: '$country'
    }
]).pretty()

您可以使用这个 mongo 查询来获取国家/地区对象

Answer 2

当您不想重复项目中的所有字段时，只需使用 $addFields 覆盖有问题的字段即可：

db.users.aggregate([
    {   "$project": {     
        "fullName": {       
            "$concat": [ "$firstName", " ", "$lastName"]     
        },
        "country": "$country"   
    }}, 
    { "$lookup": {     
        "from": "countries",     
        "localField": "country",     
        "foreignField": "_id",     
        "as": "countryInfo"   
    }},
    { "$addFields": {
        "countryInfo": {
            "$arrayElemAt": [ "$countryInfo", 0 ]
        }
    }}
])

Answer 3

你也可以使用"preserveNullAndEmptyArrays"

像这样：

 db.users.aggregate(
        [
            {   "$project": {     
                "fullName": {       
                    "$concat": [ "$firstName", " ", "$lastName"]     
                },
                "country": "$country"   
            }}, 
            { "$lookup": {     
                    "from": "countries",     
                    "localField": "country",     
                    "foreignField": "_id",     
                    "as": "countryInfo"   
            }}, 
            {"$unwind": {
                    "path": "$countryInfo",
                    "preserveNullAndEmptyArrays": true
                }
            },
        ]
    )

Answer 4

您快到了，您需要将另一个$project 阶段添加到您的管道并使用$arrayElemAt 返回数组中的单个元素。

db.users.aggregate(
    [
        {   "$project": {     
            "fullName": {       
                "$concat": [ "$firstName", " ", "$lastName"]     
            },
            "country": "$country"   
        }}, 
        { "$lookup": {     
                "from": "countries",     
                "localField": "country",     
                "foreignField": "_id",     
                "as": "countryInfo"   
        }}, 
        { "$project": { 
            "fullName": 1, 
            "country": 1, 
            "countryInfo": { "$arrayElemAt": [ "$countryInfo", 0 ] } 
        }} 
    ]
)