Mongodb aggregation pipeline how to limit a group push

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.

Solution 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 }
])

Solution 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")