MongoDB多维数组投影 [英] MongoDB multidimensional array projection

问题描述

我刚刚开始学习MongoDB，找不到解决我问题的方法.

I just started learning MongoDB and can't find a solution for my problem.

得到该文件:

> db.test.insert({"name" : "Anika", "arr" : [ [11, 22],[33,44] ] })

请注意"arr"字段，它是一个多维数组.

Please note the "arr" field which is a multidimensional array.

现在，我正在寻找一个仅返回arr [0] [1]的值(即22)的查询.我试图通过使用$ slice来实现这一点，但是我不知道如何解决第二维问题这样.

Now I'm looking for a query that returns only the value of arr[0][1] which is 22. I tried to achieve that by using $slice, however I don't know how to address the second dimension with that.

> db.test.find({},{_id:0,"arr":{$slice: [0,1]}})
{ "name" : "ha", "arr" : [ [ 11, 22 ] ] }

我也尝试过

> db.test.find({},{_id:0,"arr":{$slice: [0,1][1,1]}})
{ "name" : "ha", "arr" : [ [ 11, 22 ] ] }

所需的输出将是

22

或

{"arr":[[22]]}

谢谢

阅读评论后，我认为我已经简化了示例数据太多了，我必须提供更多信息:

After reading the comments I think that I've simplified the example data too much and I have to provide more information:

1. 馆藏中还有更多类似的文件 我已经提供了.但是它们都具有相同的结构.
2. 数组元素多于两个
3. 在现实世界中，数组包含非常长的文本(500kb-1mb)， 因此将整个数据传输到客户端非常昂贵.
4. 在聚合之前，我将通过名称"字段进行查询.只是 为了简单起见，在示例中跳过了这一点.
5. 目标索引是可变的，所以有时我需要知道 arr [0] [1]的值，下次是arr [1] [4]

1. There are many more documents in the collection like that one that I've provided. But they all have the same structure.
2. There are more array elements than just two
3. In the real world the array contains really long texts (500kb-1mb), so it is very expansive to transmit the whole data to the client.
4. Before the aggregation I will do a query by the 'name' field. Just skipped that in the example for the sake of simplicity.
5. The target indexes are variable, so sometimes I need to know the value of arr[0][1], the next time it is arr[1][4]

---

示例数据:

---

example data:

> db.test.insert({"name" : "Olivia", "arr" : [ [11, 22, 33, 44],[55,66,77,88],[99] ] })
> db.test.insert({"name" : "Walter", "arr" : [ [11], [22, 33, 44],[55,66,77,88],[99] ] })
> db.test.insert({"name" : "Astrid", "arr" : [ [11, 22, 33, 44],[55,66],[77,88],[99] ] })
> db.test.insert({"name" : "Peter",  "arr" : [ [11, 22, 33, 44],[55,66,77,88],[99] ] })

示例查询:

> db.test.find({name:"Olivia"},{"arr:"...})

推荐答案

您可以使用聚合框架:

db.test.aggregate([
    { $unwind: '$arr' },
    { $limit: 1 },
    { $project: { _id: 0, arr: 1 } },
    { $unwind: '$arr' },
    { $skip: 1 },
    { $limit: 1 }
])

返回:

{ "arr": 22 }

编辑:原始海报修改了我的解决方案以满足他的需要，并提出了以下建议:

The original poster has modified my solution to suit his needs and came up with the following:

db.test.aggregate([
    { $match: { name:"Olivia" } },
    { $project: { _id: 0,arr: 1 } },
    { $unwind: '$arr' },
    { $skip: 1 },
    { $limit:1 },
    { $unwind: "$arr" },
    { $skip: 2 },
    { $limit: 1 }
])

给定OP提供的扩展数据，此查询将导致{ arr: 77 }.请注意，需要$ skip和$ limit来选择数组层次结构中的正确元素.

This query will result in { arr: 77 } given the extended data provided by the OP. Note that $skip and $limit are needed to select the right elements in the array hierarchy.

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