Mongodb aggregation pipeline how to limit a group push

Question

I am not able to limit the amount of pushed elements in a group function with aggregation pipeline. Is this possible? Small example:

Data:

[     {         "submitted": date,         "loc": { "lng": 13.739251, "lat": 51.049893 },         "name": "first",         "preview": "my first"     },     {         "submitted": date,         "loc": { "lng": 13.639241, "lat": 51.149883 },         "name": "second",         "preview": "my second"     },     {         "submitted": date,         "loc": { "lng": 13.715422, "lat": 51.056384 },         "name": "nearpoint2",         "preview": "my nearpoint2"     } ] 

Here is my aggregation pipeline:

var pipeline = [     //I want to limit the data to a certain area     { $match: {         loc: {             $geoWithin: {                 $box: [                     [locBottomLeft.lng, locBottomLeft.lat],                     [locUpperRight.lng, locUpperRight.lat]                 ]             }         }     }},     // I just want to get the latest entries       { $sort: { submitted: -1 } },     // I group by name     {       $group: {           _id: "$name",           // get name           submitted: { $max: "$submitted" },           // get the latest date           locs: { $push: "$loc" },           // push every loc into an array THIS SHOULD BE LIMITED TO AN AMOUNT 5 or 10           preview: { $first: "$preview" }       }     },     // Limit the query to at least 10 entries.     { $limit: 10 } ]; 

How can I limit the locs array to 10 or any other size? I tried something with $each and $slice but that does not seem to work.

Answer 1

Suppose the bottom left coordinates and the upper right coordinates are respectively [0, 0] and [100, 100]. From MongoDB 3.2 you can use the $slice operator to return a subset of an array which is what you want.

db.collection.aggregate([     { "$match": {          "loc": {              "$geoWithin":  {                  "$box": [                      [0, 0],                      [100, 100]                 ]             }         }}     }},     { "$group": {          "_id": "$name",         "submitted": { "$max": "$submitted" },          "preview": { "$first": "$preview" }         "locs": { "$push": "$loc" }     }},      { "$project": {          "locs": { "$slice": [ "$locs", 5 ] },         "preview": 1,         "submitted": 1     }},     { "$limit": 10 } ]) 

Answer 2

Starting in Mongo 5.2, it's a perfect use case for the new $topN aggregation accumulator:

// { submitted: ISODate("2021-12-05"), group: "group1", value: "plop" } // { submitted: ISODate("2021-12-07"), group: "group2", value: "smthg" } // { submitted: ISODate("2021-12-06"), group: "group1", value: "world" } // { submitted: ISODate("2021-12-12"), group: "group1", value: "hello" } db.collection.aggregate([   { $group: {     _id: "$group",     top: { $topN: { n: 2, sortBy: { submitted: -1 }, output: "$value" } }   }} ]) // { _id: "group1", top: [ "hello", "world" ] } // { _id: "group2", top: [ "smthg" ] } 

This applies a $topN group accumulation that:

• takes for each group the top 2 (n: 2) elements
• top 2, as defined by sortBy: { submitted: -1 } (reversed chronological)
• and for each grouped record extracts the field value (output: "$value")