蒙戈聚合 [英] Mongo Aggregation

问题描述

我的收藏中有以下文档

{
    "_id": ObjectId("54490b8104f7142f22ecc97f"),
        "title": "Sample1",
        "slug": "samplenews",
        "cat": "sports",
        "desc": "sampletextsampletext",
        "published_date": ISODate("2014-10-23T14:06:57.0Z"),

} {
    "_id": ObjectId("54490b8104f7142f22ecc97f"),
        "title": "Sample2",
        "slug": "samplenews2",
        "category": "entertaintment",
        "desc": "sampletextsampletext",
        "published_date": ISODate("2014-10-22T14:06:57.0Z"),

} {
    "_id": ObjectId("54490b8104f7142f22ecc97f"),
        "title": "Sample3",
        "slug": "samplenews3",
        "category": "entertaintment",
        "desc": "sampletextsampletext",
        "published_date": ISODate("2014-9-22T14:06:57.0Z"),

} {
    "_id": ObjectId("54490b8104f7142f22ecc97f"),
        "title": "Sample4",
        "slug": "samplenews4",
        "category": "other",
        "desc": "sampletextsampletext",
        "published_date": ISODate("2014-10-22T14:06:57.0Z"),

}

我需要一个查询来获取每个类别的前 5 条最新消息.有什么建议吗?

I need one query to get top 5 latest news from each category.any suggestions?

推荐答案

假设您安装了最新版本的 mongodb，一种方法是:

Assuming you have the latest version of mongodb installed, one way of doing it is:

• Sort 根据 published_date 降序排列记录.
• group 基于记录的category.对于每个组，将所有记录收集到一个数组中.
• 在javascript/客户端代码中，切片每个组(类别)的前5条记录.

• Sort the records based on the published_date in descending order.
• group the records based on their category. For each group, collect all the records together in an array.
• In the javascript/client side code, slice the top 5 records, of each group(category).

$slice 在服务器端 $project 聚合管道操作符中不可用，它阻止我们在服务器端执行操作.

The $slice is not available in the server side $project aggregation pipeline operator, which holds us from performing the operation on the server side.

var result = db.collection.aggregate(
[
{$sort:{"published_date":-1}},
{$group:{"_id":"$category","values":{$push:"$$ROOT"}}}
]
).map(function(doc){
return {"category":doc._id,"records":doc.values.slice(0,5)};
});

result 变量现在将是一个文档数组.每个文档代表每个 category 并且依次有一个顶部 5 记录的数组.

The result variable will now be an array of documents. Each document representing each category and in turn having an array of top 5 records.

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