投影返回"$ arrayElemAt"的元素 [英] Project an element returned with "$arrayElemAt"

问题描述

如果我在这里混淆一​​些术语，请原谅我，但是我正在使用'$ lookup'运算符在聚合中执行联接操作，如下所示:

Forgive me if I confuse some terminology here, but I'm performing a join operation in an aggregation using the '$lookup' operator as shown here:

db.collection('items').aggregate([{$match: {}},
{
    $lookup: {
        from: 'usr',
        localField: 'usr._id',
        foreignField: '_id',
        as: '__usr'
    }
}, {
    $project: {
        info: 1,
        timestamp: 1,
        usr: {
            "$arrayElemAt": [ "$__usr", 0 ]
        }
    }
}], (err, result) => {
    res.json(result);
    db.close();
});

我正在对聚合结果进行投影，并且正在使用'$ arrayElemAt'从结果数组中提取单个'usr'匹配项. 出于明显的原因，我不想返回包含敏感信息的整个"usr"记录. 我要执行的操作是对使用'$ arrayElemAt'操作返回的元素执行投影. 我能够完成此操作的唯一方法是使用原始投影的其他投影，如下所示:

I'm performing a projection on the aggregation result, and I'm using '$arrayElemAt' to extract the single 'usr' match from the resulting array. For obvious reasons, I don't want to return the entire 'usr' record which contains sensitive information. What I'm looking to do is perform a projection on the element returned using the '$arrayElemAt' operation. The only way I've been able to accomplish this is to use an additional projection of the original projection, like so:

db.collection('items').aggregate([{$match: {}},
{
    $lookup: {
        from: 'usr',
        localField: 'usr._id',
        foreignField: '_id',
        as: '__usr'
    }
}, {
    $project: {
        info: 1,
        timestamp: 1,
        usr: {
            "$arrayElemAt": [ "$__usr", 0 ]
        }
    }
}, {
    $project: {
        info: 1,
        timestamp: 1,
        usr: {
            "username": 1
        }
    }
}], (err, result) => {
    res.json(result);
    db.close();
});

有没有一种方法可以在没有重复投影的情况下完成此任务?

Is there a way to accomplish this without the duplicated projection?

推荐答案

看来这是 still ，截至2018年8月，这是完成通过到单个嵌入式文档并对其字段进行过滤仍然是通过辅助$project管道阶段.虽然@styvane上面的 方法确实可行，并且确实很新颖，但它却增加了管道的复杂性，因此我很犹豫地接受此作为答案.

It looks like this is still, as of August 2018, the best way to accomplish the projection of a document joined via $lookup to a single embedded document and filter it's fields is still via a secondary $project pipeline stage. While the method shown by @styvane above does work, and is indeed very novel, it adds even more complexity to the pipeline so I'm hesitant to accept this as the answer.

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