聚合框架管道中的数组子集

问题描述

在上一个问题中，我想获得结果的计数组使用管道操作.如建议的那样，我使用了以下内容:

In a previous question I wanted to obtain a count of the resulting groups using pipeline operations. As suggested, I used the following:

db.test.aggregate(
    {$unwind: '$tags'}, 
    {$group:{_id: '$tags', count:{$sum:1}}},
    {$project:{tmp:{tag:'$_id', count:'$count'}}}, 
    {$group:{_id:null, total:{$sum:1}, data:{$addToSet:'$tmp'}}}
)

现在知道计数了，我想按页面显示结果，所以我只需要data的子集.我最初的想法是在$project管道中的data上使用$slice，例如:

Now having known the count, I would like to display the results by page so I would only need a subset of data. My initial thought would be using $slice on data within a $project pipeline like:

...
{$project: {data : { $slice: [20,20] }, total: 1}

但是$slice似乎不是$project的有效操作.我尝试通过以下方法解决此问题:

But it appears that $slice is not a valid operation for $project. I tried a workaround by doing:

db.test.aggregate(
    {$unwind: '$tags'}, 
    {$group:{_id: '$tags', count:{$sum:1}}},
    {$project:{tmp:{tag:'$_id', count:'$count'}}}, 
    {$group:{_id:null, total:{$sum:1}, data:{$addToSet:'$tmp'}}},
    {$unwind: '$data'},
    {$skip: 20},
    {$limit: 20}
)

但是看起来，我执行了另一个$unwind管道.是否有更好的解决方案来实现我想要做的事情?

But as it appears, I performed another $unwind pipeline. Is there a better solution to achieve what I am trying to do?

推荐答案

不幸的是，目前(在MongoDB 2.2上)没有聚合框架运算符$slice或获取数组的子集.

Unfortunately there is currently (as at MongoDB 2.2) no Aggregation Framework operator to $slice or take a subset of an array.

您将需要使用以下解决方法:

You will need to use a workaround such as:

• 您在aggregate()管道中使用$skip和$limit
• 在应用程序代码中处理结果.
• 使用 Map/Reduce
实施汇总

• your use of $skip and $limit in the aggregate() pipeline
• manipulation of the results in your application code.
• implementing the aggregation using Map/Reduce

MongoDB问题跟踪器中有一个现有的功能请求，您可以对其进行投票/观看: SERVER- 6074:在$ project 中允许$ slice运算符.

There is an existing feature request in the MongoDB issue tracker that you can upvote/watch: SERVER-6074: Allow $slice operator in $project.

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