Mongo聚合

Question

I have following documents in my collection

{
    "_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"),

}

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

Answer 1

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

• 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).

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)};
});

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.