[SO] update with positional parameter & parameterized field name

query.js

//http://stackoverflow.com/questions/42234713/updating-info-with-mongoose-array-inside-object-inside-object-dynamically/42238863#42238863
var mongoose = require('mongoose');
mongoose.connect('mongodb://localhost:27017/accounts');

const Schema = mongoose.Schema;

const championSchema = new Schema({
    champ_1: String,
    champ_2: String,
    champ_3: String
}, { _id: false });

const playerSchema = new Schema({
    history_moves: [String],
    champions: [championSchema]
}, { _id: false });

const matchSchema = new Schema({
    room: String,
    player_1_details: playerSchema,
    player_2_details: playerSchema
});

//Update Champion
matchSchema.statics.update_champ = function(room, turn, champ_num, champ_select, callback) {

    var modifier = { $set: {} };
    modifier.$set['player_' + turn + '_details.champions.$.champ_' + champ_num] = champ_select;

    var match = {};
    match['room'] = room;
    match['player_' + turn + '_details.champions.champ_' + champ_num] = { $exists: true };

    this.update(match, modifier)
        .exec(function(error) {
            if (error) {
                return callback(error);
            } else {
                return callback(null);
            }
        });
};

var Match = mongoose.model('Matches', matchSchema);

Match.update_champ("room_0.0940045412694186", 1, 3, "new_value", function(err, res) {
    if (!err) {
        console.log(err);
        return;
    }
    console.log(res);
});

/*
//insert item
var match = new Match();

match.room = "room_0.0940045412694186";

match["player_1_details"] = new Player({
    "history_moves": [],
    "champions": [{
        "champ_1": "na"
    }, {
        "champ_2": "na"
    }, {
        "champ_3": "na"
    }]
});

match["player_2_details"] = new Player({
    "history_moves": [],
    "champions": [{
        "champ_1": "na"
    }, {
        "champ_2": "na"
    }, {
        "champ_3": "na"
    }]
});

match.save(function(err, req) {
    console.log(err);
    console.log(req);
})
*/