MongoDB：使用 $cond 和 $regex 进行聚合

Question

我试图在多个阶段对数据进行分组。

目前我的查询如下所示：

db.captions.aggregate([
{$project: {
    "videoId": "$videoId",
    "plainText": "$plainText",
    "Group1": {$cond: {if: {$eq: ["plainText", {"$regex": /leave\sa\scomment/i}]}, 
        then: "Yes", else: "No"}}}}
])

我不确定是否真的可以在聚合阶段的 $cond 中使用 $regex 运算符。我非常感谢您的帮助！

提前致谢

Answer 1

更新：从 MongoDB v4.1.11 开始，您的问题似乎终于有了一个很好的解决方案，记录在此处。

原答案：

正如我在上面的评论中所写的那样，目前$regex在内部不起作用$cond。有一个开放的JIRA 票，但它是，呃，好吧，开放......

在您的具体情况下，我倾向于建议您在客户端解决该主题，除非您正在处理大量输入数据，而这些输入数据将始终只返回小子集。从您的查询来看，您似乎总是要检索刚刚分入两个结果组（“是”和“否”）的所有文档。

如果您不想或无法在客户端解决该主题，那么这里有一些使用$facet（需要 MongoDB >= v3.4）的东西 - 它既不是特别快也不是特别漂亮，但它可能会帮助您入门。

db.captions.aggregate([{
    $facet: { // create two stages that will be processed using the full input data set from the "captions" collection
        "CallToActionYes": [{ // the first stage will...
            $match: { // only contain documents...
                "plainText": /leave\sa\scomment/i // that are allowed by the $regex filter (which could be extended with multiple $or expressions or changed to $in/$nin which accept regular expressions, too)
            }
        }, {
            $addFields: { // for all matching documents...
                "CallToAction": "Yes" // we create a new field called "CallsToAction" which will be set to "Yes"
            }
        }],
        "CallToActionNo": [{ // similar as above except we're doing the inverse filter using $not
            $match: {
                "plainText": { $not: /leave\sa\scomment/i }
            }
        }, {
            $addFields: {
                "CallToAction": "No" // and, of course, we set the field to "No"
            }
        }]
    }
}, {
    $project: { // we got two arrays of result documents out of the previous stage
        "allDocuments" : { $setUnion: [ "$CallToActionYes", "$CallToActionNo" ] } // so let's merge them into a single one called "allDocuments"
    }
}, {
    $unwind: "$allDocuments" // flatten the "allDocuments" result array
}, {
    $replaceRoot: { // restore the original document structure by moving everything inside "allDocuments" up to the top
        newRoot: "$allDocuments"
    }
}, {
    $project: { // include only the two relevant fields in the output (and the _id)
        "videoId": 1,
        "CallToAction": 1
    }
}])

与聚合框架一样，它可能有助于从管道末端删除单个阶段并运行部分查询，以便了解每个阶段的作用。