mongoDB子元素上的数组长度

问题描述

我的mongo数据库中有类似以下的数据.显然array1的长度是2，我正在寻找一种使用mongo db脚本来计算这种数组的长度的方法.谢谢

  {"_id":ObjectId("65465465465465465468")，标题":{测试一下"，}，值":11.763548134011048，"array1":[{"q":{值":250}}，{"q":{值":30，}}]，"array2":[]} 

解决方案

您可以使用 $ size 聚合运算符.

鉴于您在OP中显示的文档，请使用以下命令...

  db.collection.aggregate([{$ project:{sizeOfArray1:{$ size:"$ array1"}}}]) 

...将返回:

  {"_id":ObjectId("65465465465465465468")，"sizeOfArray1":2} 

此方法对Mongo> = 2.6 的所有版本均有效.

更新:针对此评论...

您可以使用带有.的路径符号来寻址任何文档属性，以表示每个级别".因此，如果您的文档如下所示:

  {"_id":ObjectId("65465465465465465468")，标题":{测试一下"，数组":[1、2、3]}，值":11.763548134011048，"array1":[{"q":{值":250}}，{"q":{值":30，}}]，"array2":[]} 

...，则以下命令将返回 header 嵌套子文档中的 array 字段的计数:

  db.collection.aggregate([{$ project:{sizeOfHeaderArray:{$ size:"$ header.array"}}}]) 

因此，也许现在您可以看到，您将使用 header.array 来解决 header 嵌套子文档中的 array 字段./p>

希望有帮助.

I have a data similar to the below in my mongo DB. Obviously the length of array1 is two and I am looking on a way to count the length of such array with mongo db script. Thanks

{
        "_id" : ObjectId("65465465465465465468"),
        "header" : {
                "test" : "test",
        },
        "value" : 11.763548134011048,
        "array1" : [
                {
                        "q" : {
                                "value" : 250
                        }
                },
                {
                        "q" : {
                                "value" : 30,
                        }
                }
        ],
        "array2" : [ ]
}

解决方案

You can use the $size aggregation operator.

Given the document you showed in your OP, the following command ...

db.collection.aggregate(
   [
      {
         $project: {
            sizeOfArray1: { $size: "$array1" }
         }
      }
   ]
)

... will return:

{
    "_id" : ObjectId("65465465465465465468"),
    "sizeOfArray1" : 2
}

This approach is valid for all versions of Mongo >= 2.6.

Update: in response to this comment ...

You address any document attribute using path notation with a . to denote each 'level'. So, if your document looked like this:

{
        "_id" : ObjectId("65465465465465465468"),
        "header" : {
                "test" : "test",
                "array" : [1, 2, 3]
        },
        "value" : 11.763548134011048,
        "array1" : [
                {
                        "q" : {
                                "value" : 250
                        }
                },
                {
                        "q" : {
                                "value" : 30,
                        }
                }
        ],
        "array2" : [ ]
}

... then the following command would return the count of the array field inside the header nested sub document:

db.collection.aggregate(
   [
      {
         $project: {
            sizeOfHeaderArray: { $size: "$header.array" }
         }
      }
   ]
)

So, perhaps you can now see that you would address the array field inside the header nested sub document using header.array.

Hope that helps.

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