带有 $lookup 的 MongoDB 聚合仅包括(或投影)一些要从查询返回的字段

Question

在 mongo 中，在使用 $lookup 进行聚合之后，我希望请求只返回一些字段而不是整个文档。

我有以下查询:

db.somecollection.aggregate([{
    $lookup: {
        from: "campaigns",
        localField: "campId",
        foreignField: "_id",
        as: "campaign"
    }
}, {
    $unwind: "$campaign"
}, {
    $lookup: {
        from: "entities",
        localField: "campaign.clientid",
        foreignField: "_id",
        as: "campaign.client"
    }
}]);

此请求将返回给我:

{
"_id" : ObjectId("56cc7cd1cc2cf62803ebfdc7"),
"campId" : ObjectId("56c740e4479f46e402efda84"),
"articleId" : ObjectId("56c742c06094640103ba3843"),
"campaign" : {
    "_id" : ObjectId("56c740e4479f46e402efda84"),
    "clientid" : ObjectId("56c740b8479f46e402efda83"),
    "client" : [
        {
            "_id" : ObjectId("56c740b8479f46e402efda83"),
            "username" : "someusername",
            "shhh" : "somehashedpassword",
            "email" : "mail@mail.com",
        }
    ]
}

请求运行良好，但我想过滤 campaign.client 中的字段以仅获取例如 _id 和 username。有没有办法在 MongoDB aggregate 请求中执行此操作？

Answer 1

只是为了帮助其他人，@SiddhartAjmera 给出了正确的答案，我只需要为嵌套值添加双引号，例如“campaign.clientid”。

最终代码应该是:

db.somecollection.aggregate([
      {
        "$lookup": {
          "from": "campaigns",
          "localField": "campId",
          "foreignField": "_id",
          "as": "campaign"
        }
      },
      {
        "$unwind": "$campaign"
      },
      {
        "$lookup": {
          "from": "entities",
          "localField": "campaign.clientid",
          "foreignField": "_id",
          "as": "campaign.client"
        }
      },
      {
        "$project": {
          "_id": 1,
          "campId": 1,
          "articleId": 1,
          "campaign._id": 1,
          "campaign.clientid": 1,
          "campaign.client._id": 1,
          "campaign.client.username": 1
        }
      }
]);