创建具有仅不同值的行的单风

Question

我正在使用mongo查询聚合，其中有这样的数据

[
    {
        "token" : "c1",
        "class" : "bscs",
        courses : ["ITM", "CS54", "KHE"]
    }
]

我正在使用 $ unwind 来中断课程

[
    {
        "token" : "c1",
        "class" : "bscs",
        courses : "ITM"
    },
    {
        "token" : "c1",
        "class" : "bscs",
        courses : "CS54"
    },
    {
        "token" : "c1",
        "class" : "bscs",
        courses : "KHE"
    }
]

我想要的是，它应该像：-

[
    {
        "token" : "c1",
        "class" : "bscs",
        courses : "ITM"
    },
    {
        "token" : "",
        "class" : "",
        courses : "CS54"
    },
    {
        "token" : "",
        "class" : "",
        courses : "KHE"
    }
]

因为我有一个表结构，需要显示数据，这非常复杂，并且在那里显示了来自不同查询的许多数据，所以我无法更新它。如果它是这样，那么我可以在表中显示为

我的汇总是

[   {.....}, // others projections and matches
   { 
    $unwind: {
     path: '$courses'
    }
   }
]

Answer 1

在运行$unwind时可以使用includeArrayIndex选项，然后在索引不等于0的情况下使用$cond设置空字符串

db.collection.aggregate([
    {
        $unwind: {
            path: "$courses",
            includeArrayIndex: "index"
        }
    },
    {
        $project: {
            token: { $cond: [ { $eq: [ "$index", 0 ] }, "$token", "" ] },
            class: { $cond: [ { $eq: [ "$index", 0 ] }, "$class", "" ] },
            courses: 1
        }
    }
])

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