如何在MongoDB中查找数组大小在一定范围内的文档? [英] How to find a document with array size within a range in MongoDB?

问题描述

给出以下文档结构:

{
  _id : Mongo ID,
  data : [someData1, someData2, ..., someDataN]
}

我想获取所有data大小在a和b之间的文档(严格的正整数，请考虑a < b始终为true).
我首先想到使用 $size ，但是Mongo文档指出:

I want to get all documents which data size is between a and b (strict positive integers, consider a < b is always true).
I first thought about using $size, but the Mongo doc states:

$size不接受值的范围.要根据元素数量不同的字段选择文档，请创建一个计数器字段，在将元素添加到字段时会增加该字段.

$size does not accept ranges of values. To select documents based on fields with different numbers of elements, create a counter field that you increment when you add elements to a field.

(强调我的)

如何在不使用计数器的情况下检索长度在a和b之间的data数组的所有文档?

How to retrieve all the documents with a data array of length between a and b without using a counter?

相关但未解决:

• 此答案建议使用计数器
• 此问题，询问如何查询内部长度 得到很多答案
• 此答案，建议使用:{ 'data.' + b : { $exists : true } }，但我不知道它的可靠性以及如何将其应用于范围
• 此问题有关如何查找具有最小数组长度的文档

• This answer which suggests using a counter
• This question which asks how to query an internal length and got many answers
• This answer which suggests to use: { 'data.' + b : { $exists : true } } but I don't know how reliable it is and how it could apply to a range
• This question about how to find documents with a minimal array length

推荐答案

您可以使用$exists运算符方法，在查询中包括多个术语并以编程方式进行构建:

You can use the $exists operator approach by including multiple terms in your query and building it up programmatically:

var startKey = 'data.' + (a-1);
var endKey = 'data.' + b;
var query = {};
query[startKey] = {$exists: true};
query[endKey] = {$exists: false};
db.test.find(query);

例如，对于a=1和b=3，您最终得到的query对象看起来像:

So for a=1 and b=3, for example, you end up with a query object that looks like:

{
    "data.0" : {
        "$exists" : true
    },
    "data.3" : {
        "$exists" : false
    }
}

第一部分确保至少有a个元素，第二部分确保不超过b个元素.

The first part ensures there are at least a elements, and the second part ensures there are no more than b elements.

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