$lookup 阶段内的 MongoDB 聚合 $elemMatch

Question

我有一个如下所示的 MongoDB 集合：

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

我想使用之前聚合阶段的结果，现在使用$lookup 阶段查找播放器出现的所有文档：

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

$lookup 阶段的输入如下所示：

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

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

我也尝试过使用let 属性，这给了我相同的结果。 我做错了什么？

Answer 1

使用您提供的文件。我相信这可能对你有用：

我使用$lookup 对包含playerId 的集合进行连接，这会创建一个名为field 的array。然后我使用$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。

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 ] 
      } } }
    ])

我故意让输出变得冗长..

/* 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
}