Mongodb 聚合管道限制 $lookup 字段答案

【问题标题】：Mongodb Aggregation Pipeline limit $lookup fieldsMongodb 聚合管道限制 $lookup 字段
【发布时间】：2018-08-06 11:18:20
【问题描述】：

我有两个集合，我试图将它们聚合在一起以产生以下所需的输出，但我正在努力解决如何限制查找返回的数组中的字段。任何帮助将不胜感激。

project
{
    _id: ObjectId("ABC")    
    projectName: "Project One"
}

user
{
    _id: ObjectId("...")
    email: "test1@test.co.uk",
    firstname: "Test1",
    lastname: "Record",
    project: [{projectId: ObjectId("ABC")}]
},
{
    _id: ObjectId("...")
    email: "test2@test.co.uk",
    firstname: "Test2",
    lastname: "Record",
    project: [{projectId: ObjectId("ABC")}, {projectId: ObjectId("DEF")}]
}

想要的输出

{
    projectName: "Project One"
    userDetail: [{firstname: "Test1",lastname: "Record"}, 
            {firstname: "Test2",lastname: "Record"}]        
}

目前的工作

db.project.aggregate([{$match: {_id: ObjectId("ABC")}} 
,  {$lookup: {
                from: "user",
                localField: "_id",
                foreignField: "project.projectId",
                as: "userDetail"
            }
} 
, {$project: {_id: 1, projectName: 1}}
]).pretty()

【问题讨论】：

标签： mongodb aggregation-framework

【解决方案1】：

使用$map 在$project 阶段映射您选择的字段。

{
  "$project":{
    "projectName":1,
    "userDetail":{
      "$map":{
        "input":"$userDetail",
        "as":"ud",
        "in":{
          "firstname":"$$ud.firstname",
          "lastname":"$$ud.lastname"
        }
      }
    }
  }
}

【讨论】：

【解决方案2】：

db.project.aggregate([{$match: {_id: ObjectId("ABC"),}} , 
{$lookup: {
            from: "user",
            localField: "_id",
            foreignField: "project.projectid",
            as: "userDetail"
        }
}, {$project: {_id: 1, projectName:1,"userDetail.firstname":1,"userDetail.lastname":1}}
]).pretty()

【讨论】：