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:

Answer 1

You can achieve this by passing $slice operator directly at the $push.

  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: {
              $slice: 10
            }
        },
        < --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
}
];