MongoDB聚合中$lookup阶段内的$elemMatch

Question

MongoDB聚合中$lookup阶段内的$elemMatch

mongodbmongodb-queryaggregation-framework

4

我有一个 MongoDB 集合，看起来是这样的：

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

我想使用先前聚合阶段的结果，现在使用 $lookup 阶段查找所有出现玩家的文档:

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

我的$lookup阶段的输入如下所示：

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

这个查询在field中返回一个空数组。看起来$uuid没有被正确地评估，因为如果我用硬编码值（例如A）替换$uuid，这个查询会返回结果。

我也尝试使用let属性，但结果相同。我做错了什么？

- Joba

1

你能否从集合中发布带有示例文档的内容？ - Saravana

我的帖子顶部包含示例文件。 - Joba

1个回答

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- user9251303 · Accepted Answer

使用您提供的文档，我认为以下内容可能适用于您：

我使用了$lookup来进行连接到保存playerId的集合，从而创建了一个名为field的array。然后我使用$unwind从field和player中提取所有数组元素。最后，我使用$cond来交叉检查是否两个值匹配。

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 ] 
      } } }
    ])

我特意保留了冗长的输出。

/* 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
}