仅返回投影数组子文档中的特定字段 [英] Return only specific fields from projection array sub-document

问题描述

我正在 $ 和 $elemMatch 预测.我试图弄清楚如何只返回一个子集 投影数组的字段，但我似乎无法弄清楚.

I'm looking at MongoDB's documentation on the $ and $elemMatch projections. I'm trying to figure out how to return only a subset of a projection array's fields, but I cannot seem to figure it out.

相关帖子:

• 我不尝试执行 $ slice mongodb聚合框架中的a>-获取嵌套数组的第一个文档字段.

• I am not trying to perform a $slice from mongodb aggregation framework - Fetch first document's field of the nested array.

我也不是试图从仅在mongo投影中返回数组值中的子文档吗?仍然需要顶部文档中的某些字段.

Nor am I trying to flatten the sub-document from Return only array value in mongo projection because I still want some fields from the top document.

说我在test集合中有以下文档:

Say I have the following documents in the test collection:

{
    "_id": "A",
    "array": [
        {"key": 1, "name": "foo", "data": {}},
        {"key": 2, "name": "bar", "data": {}}
    ],
    "extra": {}
},
{
    "_id": "B",
    "array": [
        {"key": 3, "name": "spam", "data": {}},
        {"key": 4, "name": "eggs", "data": {}}
    ],
    "extra": {}
}

我实际上要执行的查询是:

The query I effectively want to perform is:

db.test.findOne({"array.key": 1}, {"array.$.name": 1, "extra": 1})

我希望它只返回name key是1的数组.例如，

Which I would expect it to only return name under the sub-document in the array where key was 1. E.g.,

{
    "_id": "A",
    "array": [
        {"name": "foo"}
    ],
    "extra": {}
}

但是，如果我执行该查询，则会得到以下信息:

But if I perform that query, I get this instead:

{
    "_id": "A",
    "array": [
        {"key": 1, "name": "foo", "data": {}}
    ],
    "extra": {}
}

与执行查询相同:

db.test.findOne({"array.key": 1}, {"array.$": 1, "extra": 1})

我还尝试了以下操作，结果相同:

I've also tried the following which results in the same:

db.test.findOne({"array.key": 1}, {"array.$": 1, "array.name": 1, "extra": 1})

是否有一种方法只返回array.$的字段的子集 而不是整个子文档?

Is there a way to only return a subset of the fields for array.$ instead of the whole sub-document?

推荐答案

您是否专门尝试在不使用聚合的情况下做到这一点?

Are you specifically trying to do this without using aggregate?

db.test.aggregate([{$unwind:"$array"},
                   {$match:{"array.key":1}},
                   {$project:{"array.name":1, extra:1}}])

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