Limiting number of different kind of results in mongoDB aggregation

Question

I would like to write an aggregation that take the following documents:

{ type: "dog", name: "Charlie" }
{ type: "dog", name: "Felix" }
{ type: "dog", name: "John" }
{ type: "cat", name: "Tum" }

And returns up to 2 of each kind, not grouped in any separate way:

{ type: "dog", name: "Charlie" }
{ type: "dog", name: "Felix" }
{ type: "cat", name: "Tum" }

Meaning just up to two cats plus up to two dogs. Does grouping and limiting the way to go here? If so - how?

Answer 1

You can group the documents by type , create a list of the names per group, $project the list to only have two elements with $slice and then flatten the list using $unwind , something like the following:

Model.aggregate([
    { "$group": {
        "_id": "$type",
        "data": { "$push": "$name" }
    } },
    { "$project": {
        "type": "$_id",
        "name": { "$slice": ["$data", 2] }
    } },
    { "$unwind": "$name" }
]).exec(callback);