• Data type conversion in MongoDB


    [问题]

    I have a collection called Document in MongoDB. Documents in this collection have a field called CreationDate stored in ISO date type. My task is to count the number of documents created per day and sort by the number asynchronously. The output format is required to be [{_id:'yyyy-MM-dd', cnt:x}]. I tried to use aggregation framework as below.

    db.Document.aggregate(

        , {$project: {_id:1, Year:{$year:'$CreationDate'}, Month:{$month:'$CreationDate'}, Date:{$dayOfMonth:'$CreationDate'}}}

        , {$group: {_id:{$concat:['$Year', '-', '$Month', '-', '$Date']}, cnt:{$sum:1}}}

        , {$sort:{'cnt':-1}}

    );

    The code gives me error as below:

    $concat only supports strings, not NumberInt32

    I understand this is because $year, $month and $dayOfMonth all return number. It's possible to compose the _id field as an object and re-format it in the desired format in application level.

    But from technical perspective, I have two questions:

    1. How to convert a number to string in MongoDB shell? In this case, output of $year can then be converted to string and used in $concat.
    2. Is there a better way to format ISODate output to various date formats? In many cases, we only need certain part of an ISODate, for example: the date component or the time portion. Is there any MongoDb inbuilt operators to achieve this?

    Thanks in advance for any advice.

     [回答]

    You can do this with $concat but first you need to convert to a string via $substr, also handling the double digit case:

    db.Document.aggregate([ 
        { "$group": { 
            "_id":{ 
                "$concat": [
                     { "$substr": [ { "$year": "$CreationDate" }, 0, 4 ] },
                     "-",
                     { "$cond": [
                         { "$gt": [ { "$month": "$CreationDate" }, 9 ] },
                         { "$substr": [ { "$month": "$CreationDate" }, 0, 2 ] },
                         { "$concat": [
                             "0",
                             { "$substr": [ { "$month": "$CreationDate" }, 0, 1 ] },
                         ]},
                     ]},
                     "-",
                     { "$cond": [
                         { "$gt": [ { "$dayOfMonth": "$CreationDate" }, 9 ] },
                         { "$substr": [ { "$dayOfMonth": "$CreationDate" }, 0, 2 ] },
                         { "$concat": [
                             "0",
                             { "$substr": [ { "$dayOfMonth": "$CreationDate" }, 0, 1 ] },
                         ]}
                     ]}
                 ]
             },
             { "cnt": { "$sum": 1 } }
        }}
        { "$sort":{ "cnt" :-1 }}
    ]);

    Possibly better is to just use date math instead, this returns an epoch timestamp value, but it is easy to work into a date object in post processing:

    db.Document.aggregate([
        { "$group": {
            "_id": {
                "$subtract": [
                    { "$subtract": [ "$CreationDate", new Date("1970-01-01") ] },
                    { "$mod": [
                        { "$subtract": [ "$CreationDate", new Date("1970-01-01") ] },
                        1000 * 60 * 60 * 24
                    ]}
                ]
            },
            "cnt": { "$sum": 1 }
        }},
        { "$sort": { "cnt": -1 } }
    ])

    来自: https://stackoverflow.com/questions/25176855/data-type-conversion-in-mongodb

  • 相关阅读:
    Flex 布局:语法
    Sublime Text常用快捷键
    WebStorm快捷键操作
    获取token
    Oracle杂记
    YKT文件解析
    杂记_ 关键字
    Python Web 性能和压力测试 multi-mechanize
    详细介绍windows下使用python pylot进行网站压力测试
    python文件和目录操作方法大全
  • 原文地址:https://www.cnblogs.com/time-is-life/p/9328688.html
Copyright © 2020-2023  润新知