mongodb 使用组计数获取 $group 结果

Obtaining $group result with group count

提问by MervS

Assuming I have a collection called "posts" (in reality it is a more complex collection, posts is too simple) with the following structure:

假设我有一个名为“posts”的集合（实际上它是一个更复杂的集合，posts 太简单了）具有以下结构：

> db.posts.find()

{ "_id" : ObjectId("50ad8d451d41c8fc58000003"), "title" : "Lorem ipsum", "author" : 
"John Doe", "content" : "This is the content", "tags" : [ "SOME", "RANDOM", "TAGS" ] }

I expect this collection to span hundreds of thousands, perhaps millions, that I need to query for posts by tags and group the results by tag and display the results paginated. This is where the aggregation framework comes in. I plan to use the aggregate() method to query the collection:

我希望这个集合跨越数十万，也许数百万，我需要按标签查询帖子并按标签对结果进行分组并显示分页的结果。这就是聚合框架的用武之地。我计划使用aggregate() 方法来查询集合：

db.posts.aggregate([
  { "$unwind" : "$tags" },
  { "$group" : {
      _id: { tag: "$tags" },
      count: { $sum: 1 }
  } }
]);

The catch is that to create the paginator I would need to know the length of the output array. I know that to do that you can do:

问题是要创建分页器，我需要知道输出数组的长度。我知道要做到这一点，您可以：

db.posts.aggregate([
  { "$unwind" : "$tags" },
  { "$group" : {
      _id: { tag: "$tags" },
      count: { $sum: 1 }
  } }
  { "$group" : {
      _id: null,
      total: { $sum: 1 }
  } }
]);

But that would discard the output from previous pipeline (the first group). Is there a way that the two operations be combined while preserving each pipeline's output? I know that the output of the whole aggregate operation can be cast to an array in some language and have the contents counted but there may be a possibility that the pipeline output may exceed the 16Mb limit. Also, performing the same query just to obtain the count seems like a waste.

但这会丢弃先前管道（第一组）的输出。有没有办法在保留每个管道的输出的同时将这两个操作结合起来？我知道整个聚合操作的输出可以转换为某种语言的数组并计算内容，但管道输出可能会超过 16Mb 的限制。此外，仅仅为了获得计数而执行相同的查询似乎是一种浪费。

So is obtaining the document result and count at the same time possible? Any help is appreciated.

那么是否可以同时获取文档结果和计数呢？任何帮助表示赞赏。

回答by Chien-Wei Huang

1. Use $projectto save tagand countinto tmp
2. Use $pushor addToSetto store tmpinto your datalist.

1. 使用$project保存tag和count成tmp
2. 使用$push或addToSet存储tmp到您的data列表中。

Code:

代码：

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'}}}
)

Output:

输出：

{
    "result" : [
            {
                    "_id" : null,
                    "total" : 5,
                    "data" : [
                            {
                                    "tag" : "SOME",
                                    "count" : 1
                            },
                            {
                                    "tag" : "RANDOM",
                                    "count" : 2
                            },
                            {
                                    "tag" : "TAGS1",
                                    "count" : 1
                            },
                            {
                                    "tag" : "TAGS",
                                    "count" : 1
                            },
                            {
                                    "tag" : "SOME1",
                                    "count" : 1
                            }
                      ]
              }
      ],
      "ok" : 1
}

回答by Ross

I'm not sure you need the aggregation framework for this other than counting all the tags eg:

除了计算所有标签之外，我不确定您是否需要聚合框架，例如：

db.posts.aggregate(
  { "unwind" : "$tags" },
  { "group" : {
      _id: { tag: "$tags" },
      count: { $sum: 1 }
  } }
);

For paginating through per tag you can just use the normal query syntax - like so:

对于每个标签的分页，您可以使用普通的查询语法 - 如下所示：

db.posts.find({tags: "RANDOM"}).skip(10).limit(10)