MongoDB聚合$ elem在$ lookup阶段匹配 [英] MongoDB aggregation $elemMatch inside $lookup stage

问题描述

我有一个看起来像这样的MongoDB集合:

I have a MongoDB collection that is looking like this:

{
    players: [
        {uuid: "A"},
        {uuid: "B"}
    ]
},
{
    players: [
        {uuid: "A"},
        {uuid: "C"}
    ]
},
{
    players: [
        {uuid: "D"},
        {uuid: "E"}
    ]
}

我想使用上一个聚合阶段的结果，现在使用 $ lookup 阶段查找播放器出现的所有文档:

I want to use results of a previous aggregation stage and now find all documents where a player shows up, using a $lookup stage:

from: "collection",
pipeline: [
    {
        $match: {
            players: {
                $elemMatch: {
                    uuid: "$playerId" 
                }
            }
            //using "players.uuid": "$playerId" doesn't work either
        }
    }
],
as: "field"

我的 $ lookup 阶段的输入如下:

{ 
    "playerId" : "A"
}
{ 
    "playerId" : "B"
}
{ 
    "playerId" : "C"
}

此查询在 field 中返回一个空数组.似乎 $ uuid 没有得到正确的评估，因为如果我将 $ uuid 与硬编码的值(例如 A )交换，此查询将返回结果.

This query returns an empty array in field. It seems like $uuid is not getting evaluated correctly, because if I exchange $uuid with a hardcoded value (e.g. A), this query returns results.

我也尝试过使用 let 属性，这给了我相同的结果.我在做什么错了?

I have also tried using the let property, this gave me the same result. What am I doing wrong?

推荐答案

使用您提供的文档.我相信这可能对您有用:

Using the documents you've provided. I believe this might work for you:

我已经使用 $ lookup 对包含 playerId 的集合进行了联接，该集合创建了一个名为的 array 字段.然后，我使用 $ unwind 从 field 和 player 中提取所有数组元素.最后，我使用 $ cond 来交叉检查两个值是否匹配.

I've used $lookup to do a join onto the collection which holds the playerId, which creates an array called field. I then use $unwind to extract all the array elements from both field and player. Finally I use $cond to crosscheck if both values match.

db.getCollection('foo').aggregate([
{ $lookup : {
    from: "bar",
    localField: "players.uuid",
    foreignField: "playerId",
    as: "field"
    } },
    { $unwind : "$players" },
    { $unwind : "$field" },
    { $project : { 
      "players": 1, 
      "field" : 1, 
      "isMatch": { 
      "$cond": [ { "$eq": ["$players.uuid", "$field.playerId"] }, 1, 0 ] 
      } } }
    ])

我特意留了输出详细信息.

I've purposely left the output verbose..

/* 1 */
{
    "_id" : ObjectId("5a7f534b337e8d2b97ff2ffb"),
    "players" : {
        "uuid" : "A"
    },
    "field" : {
        "_id" : ObjectId("5a7f5374337e8d2b97ff2ffe"),
        "playerId" : "A"
    },
    "isMatch" : 1.0
}

/* 2 */
{
    "_id" : ObjectId("5a7f534b337e8d2b97ff2ffb"),
    "players" : {
        "uuid" : "A"
    },
    "field" : {
        "_id" : ObjectId("5a7f539b337e8d2b97ff2fff"),
        "playerId" : "B"
    },
    "isMatch" : 0.0
}

/* 3 */
{
    "_id" : ObjectId("5a7f534b337e8d2b97ff2ffb"),
    "players" : {
        "uuid" : "B"
    },
    "field" : {
        "_id" : ObjectId("5a7f5374337e8d2b97ff2ffe"),
        "playerId" : "A"
    },
    "isMatch" : 0.0
}

/* 4 */
{
    "_id" : ObjectId("5a7f534b337e8d2b97ff2ffb"),
    "players" : {
        "uuid" : "B"
    },
    "field" : {
        "_id" : ObjectId("5a7f539b337e8d2b97ff2fff"),
        "playerId" : "B"
    },
    "isMatch" : 1.0
}

/* 5 */
{
    "_id" : ObjectId("5a7f5356337e8d2b97ff2ffc"),
    "players" : {
        "uuid" : "A"
    },
    "field" : {
        "_id" : ObjectId("5a7f5374337e8d2b97ff2ffe"),
        "playerId" : "A"
    },
    "isMatch" : 1.0
}

/* 6 */
{
    "_id" : ObjectId("5a7f5356337e8d2b97ff2ffc"),
    "players" : {
        "uuid" : "A"
    },
    "field" : {
        "_id" : ObjectId("5a7f53a8337e8d2b97ff3000"),
        "playerId" : "C"
    },
    "isMatch" : 0.0
}

/* 7 */
{
    "_id" : ObjectId("5a7f5356337e8d2b97ff2ffc"),
    "players" : {
        "uuid" : "C"
    },
    "field" : {
        "_id" : ObjectId("5a7f5374337e8d2b97ff2ffe"),
        "playerId" : "A"
    },
    "isMatch" : 0.0
}

/* 8 */
{
    "_id" : ObjectId("5a7f5356337e8d2b97ff2ffc"),
    "players" : {
        "uuid" : "C"
    },
    "field" : {
        "_id" : ObjectId("5a7f53a8337e8d2b97ff3000"),
        "playerId" : "C"
    },
    "isMatch" : 1.0
}

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