MongoDB条件关系查询

MongoDB 4.2.2

我有两个集合投票和投票，我需要从“投票”集合中抓取字段

选民选择

，并将其添加到投票中，但前提是某些条件匹配

投票id

&

选民id

用户：

民意测验：

{
"_id" : ObjectId("5f87d988ddae726a3dbe5459"),
"name" : "RedVsWhite"
},
{
"_id" : ObjectId("5f51408ffc1d0437fa31d6f7"),
"name" : "ApplesVsOrange",
"total_votes" : "0",
}

投票：

{
"_id" : ObjectId("5f864addddae726a3dbe53de"),
"voter_id" : ObjectId("5f867e0d126ddbde24d6ee73"), //this is the id of the person
"poll_id" : ObjectId("5f87d988ddae726a3dbe5459"), //this is the poll id
"voter_selection" : "red"
}

我需要这个结果：

{
"_id" : ObjectId("5f87d988ddae726a3dbe5459"),
"name" : "RedVsWhite",
"total_votes" : "0",
"voter_selection" : red // show this if it finds votes with this _id and also if user_id match & voter_id
},
{
"_id" : ObjectId("5f51408ffc1d0437fa31d6f7"),
"name" : "ApplesVsOrange",
"total_votes" : "0"
/// dont show voter_selection if nothing match with voter_id and poll_id
}

这个查询可以工作，但问题是如果没有匹配项，它将不会显示轮询行，我需要的是即使没有匹配项也获取所有轮询，但如果有匹配项，则将字段

选民选择添加到行中

db.getCollection("Polls").aggregate(
    [
        {
            "$lookup": {
                "localField": "_id",
                "from": "VotesRecords",
                "foreignField": "poll_id",
                "as": "VotesRecords"
            }
        },
        {
            "$unwind": {
                "path": "$VotesRecords",
                "preserveNullAndEmptyArrays": true
            }
        },
        {
            "$match": {
                "VotesRecords.voter_id": ObjectId("5f867e0d126ddbde24d6ee73")
            }
        },
        {
            "$project": {
                "_id": "$_id",
                "name": "$name",
                "voter_selection": "$VotesRecords.voter_selection"
            }
        }
    ]
);


$group
按null键，并将根的所有元素组成一个数组
$lookup
与
VotesRecords
加入集合并获取用户详细信息
$unwind
解构
VotesRecords
数组
$project
在
根
数组上迭代循环
$map
，检查条件是否匹配
轮询id
，然后使用

$unwind
解构根数组
$replaceWith
替换根目录中的
对象




第二种方法，您可以在管道之后添加以下更改


$project
显示
投票者选择
如果
投票者id
匹配，否则
$$REMOVE
$group
通过
\u id
并使用
$mergeObjects
$replaceWith
替换根对象

//聚合看起来是正确的。您只是缺少了“total_Voces”：“$total_Voces”在您的$project阶段。$total_Voces只是民意调查收集的一个示例
db.getCollection("Polls").aggregate(
    [
        {
            "$lookup": {
                "localField": "_id",
                "from": "VotesRecords",
                "foreignField": "poll_id",
                "as": "VotesRecords"
            }
        },
        {
            "$unwind": {
                "path": "$VotesRecords",
                "preserveNullAndEmptyArrays": true
            }
        },
        {
            "$match": {
                "VotesRecords.voter_id": ObjectId("5f867e0d126ddbde24d6ee73")
            }
        },
        {
            "$project": {
                "_id": "$_id",
                "name": "$name",
                "voter_selection": "$VotesRecords.voter_selection"
            }
        }
    ]
);

db.getCollection("Polls").aggregate([
  {
    $group: {
      _id: null,
      root: { $push: "$$ROOT" }
    }
  },
  {
    $lookup: {
      from: "VotesRecords",
      as: "VotesRecords",
      pipeline: [{ $match: { voter_id: ObjectId("5f867e0d126ddbde24d6ee73") } }]
    }
  },
  {
    "$unwind": {
      "path": "$VotesRecords",
      "preserveNullAndEmptyArrays": true
    }
  },
  {
    $project: {
      root: {
        $map: {
          input: "$root",
          in: {
            $mergeObjects: [
              "$$this",
              {
                $cond: [
                  { $eq: ["$$this._id", "$VotesRecords.poll_id"] },
                  { voter_selection: "$VotesRecords.voter_selection" },
                  {}
                ]
              }
            ]
          }
        }
      }
    }
  },
  { $unwind: "$root" },
  { $replaceWith: "$root" }
])

  // <= skipping your pipelines here
  {
    "$project": {
      "_id": "$_id",
      "name": "$name",
      "total_votes": 1,
      "voter_selection": {
        $cond: [
          { $eq: ["$VotesRecords.voter_id", ObjectId("5f867e0d126ddbde24d6ee73")] },
          "$VotesRecords.voter_selection",
          "$$REMOVE"
        ]
      }
    }
  },
  {
    $group: {
      _id: "$_id",
      root: { $mergeObjects: "$$ROOT" }
    }
  },
  { $replaceWith: "$root" }