Mongodb 聚合管道限制 $lookup 字段
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【中文标题】Mongodb 聚合管道限制 $lookup 字段【英文标题】：Mongodb Aggregation Pipeline limit $lookup fields 【发布时间】：2018-08-06 11:18:20 【问题描述】：我有两个集合，我试图将它们聚合在一起以产生以下所需的输出，但我正在努力解决如何限制查找返回的数组中的字段。任何帮助将不胜感激。
project
_id: ObjectId("ABC")
projectName: "Project One"
user
_id: ObjectId("...")
email: "test1@test.co.uk",
firstname: "Test1",
lastname: "Record",
project: [projectId: ObjectId("ABC")]
,
_id: ObjectId("...")
email: "test2@test.co.uk",
firstname: "Test2",
lastname: "Record",
project: [projectId: ObjectId("ABC"), projectId: ObjectId("DEF")]
想要的输出
projectName: "Project One"
userDetail: [firstname: "Test1",lastname: "Record",
firstname: "Test2",lastname: "Record"]
目前的工作
db.project.aggregate([$match: _id: ObjectId("ABC")
, $lookup:
from: "user",
localField: "_id",
foreignField: "project.projectId",
as: "userDetail"
, $project: _id: 1, projectName: 1
]).pretty()
【问题讨论】：
【参考方案1】：使用$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】：
【参考方案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】：
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MongoDB 聚合管道（Aggregation Pipeline）