node.jsでmongooseを使っている際、純粋なgroup メソッドを利用する。

mongoose-group

var mongoose = require("mongoose");
var DBName = "";

M.prototype.connection = mongoose.createConnection("mongodb://127.0.0.1/" + DBName, callback);
var database = M.connection.db
var collection = database.collection("datas");

// keys (object), condition (object), initial (object), reduce (function), finalize (function), [options] (object), callback
collection.group({"id":true}, null, {}, function (curr, result) {
        result._id = curr._id;
}, function(err, results){
    if (err){
        console.log(err);
        throw err;
    }
    console.log(results);
});