MongoDB嵌套组?

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我正在尝试在mongodb中实现嵌套的组查询，并且在尝试添加外部组时遇到麻烦.给出以下(简化的)数据文档:

I'm trying to implement a nested group query in mongodb and I'm getting stuck trying to add the outer group by. Given the below (simplified) data document:

{ "timestamp" : ISODate(), "category" : "movies", "term" : "my movie" }

我正在尝试获取所有类别的列表，并且类别中应该包含最多的术语.我希望我的输出是这样的:

I'm trying to achieve a list of all categories and within the categories there should be the top number of terms. I would like my output something like this:

[ { category: "movies", terms: [ { term: "movie 1", total: 5000 }, { term: "movie 2", total: 200 } ... ] }, { category: "sports", terms: [ { term: "football 1", total: 4000 }, { term: "tennis 2", total: 250 } ... ] }, ]

我的内部小组"如下所示，并将在所有类别中排在前5名:

My 'inner group' is as shown below, and will get the top 5 for all categories:

db.collection.aggregate([ { $match : { "timestamp": { $gt: ISODate("2014-08-27") } } }, { $group : { _id : "$term", total : { $sum : 1 } } }, { $sort : { total : -1 } }, { $limit: 5 } ]); // Outputs: { "_id" : "movie 1", "total" : 943 } { "_id" : "movie 2", "total" : 752 }

我将如何实施外部小组"?

How would I go about implementing the 'outer group'?

此外，有时上述汇总返回一个空值(并非所有文档都具有术语值).如何忽略空值?

Additionally sometimes the above aggregate]ion returns a null value (not all documents have a term value). How do I go about ignoring the null values?

预先感谢

推荐答案

在这种情况下，您将需要两个组.第一组生成一个文档流，每个术语和类别每个文档一个:

You will need two groups in this case. The first group generates a stream of documents with one document per term and category:

{ $group : { _id : { category: "$category", term: "$term", }, total: { $sum : 1 } } }

第二小组随后将使用 $ push 运算符，将类别合并到一个数组中:

A second group will then merge all documents with the same term into one, using the $push operator to merge the categories into an array:

{ $group : { _id : "$_id.category", terms: { $push: { term:"$_id.term", total:"$total" } } } }