使用elemMatch进行MongoDB查询以获取嵌套数组数据 [英] MongoDB query with elemMatch for nested array data

问题描述

我有一个包含嵌套数组数据的文档.我无可救药地尝试使用$elemMatch过滤数据，但我无法弄清为什么它不起作用.

I have a document containing nested array data. I tried hopelessly to filter the data using $elemMatch but I cannot figure out why it is not working.

{
'id' : 1,
'name' : 'test',
'modules' : [
    {
        name: 'foo',
        mandatory: false,
        group: [
            {
                name: g1
            }]
    },
    {
        name: 'bar',
        mandatory: false,
        group: [
            {
                name: g2
            }]
    }]
}

我尝试使用此查询:

db.test.find(
{
  modules: {
            $elemMatch: {
                 name: "foo",
            }
  }
}

但是它一直返回所有模块.如果我使用mandatory: true，则不返回任何内容，这似乎表明它可以工作.知道我在做什么错吗?谢谢！

But it keeps returning all the modules. If I use mandatory: true it returns nothing, which seems to indicates it works. Any idea what am I doing wrong? Thanks!

推荐答案

您的查询只是返回所有包含modules元素的文档，其中name == 'foo'.要使用 $elemMatch 来过滤输出，您需要在find调用的投影参数中使用它，而不是查询的一部分:

Your query is simply returning all docs that contain a modules element where name == 'foo'. To use $elemMatch to filter the output, you need to use it in the projection argument of the find call instead of part of the query:

db.test.find({}, {modules: {$elemMatch: {name: 'foo'}}})

要结合这两个概念，可以使用$引用查询中匹配的数组元素的索引:

To combine both concepts, you can reference the index of the array element matched in the query with $:

db.test.find({modules: {$elemMatch: {name: 'foo'}}}, {'modules.$': 1})

这两种方法都会返回:

{
  "_id": ObjectId("..."),
  "modules": [
    {
      "name": "foo",
      "mandatory": false,
      "group": [
        {
          "name": "g1"
        }
      ]
    }
  ]
}

如果您需要输出中包含其他字段，请将其添加到投影对象(例如name: 1).

If you need other fields included in the output, add them to the projection object (e.g. name: 1).

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