var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var GroupSchema = new Schema({
    title: String,
    group_id: String,
    us: [{
        api_id: String
    }],
    params: [{
        key: String
    }]
});

module.exports = mongoose.model('groups', GroupSchema);

{
"_id" : ObjectId("59dc0891f9d07a42ecda4e55"),
"title" : "Grupo 2",
"group_id" : "1",
"params" : [],
"us" : [
    {
        "api_id" : "3",
        "_id" : ObjectId("59dc0891f9d07a42ecda4e56")
    }
],
"__v" : 0
}

> { MongoError: E11000 duplicate key error collection: tccdb.groups index: users.user_1 dup key: { : null }
    at Function.MongoError.create (C:UsersChristianDesktopservidortcc1node_modulesmongodb-coreliberror.js:31:11)
    at toError (C:UsersChristianDesktopservidortcc1node_modulesmongodblibutils.js:139:22)
    at C:UsersChristianDesktopservidortcc1node_modulesmongodblibcollection.js:669:23
    at handleCallback (C:UsersChristianDesktopservidortcc1node_modulesmongodblibutils.js:120:56)
    at C:UsersChristianDesktopservidortcc1node_modulesmongodblibbulkunordered.js:465:9
    at handleCallback (C:UsersChristianDesktopservidortcc1node_modulesmongodblibutils.js:120:56)
    at resultHandler (C:UsersChristianDesktopservidortcc1node_modulesmongodblibbulkunordered.js:413:5)
    at C:UsersChristianDesktopservidortcc1node_modulesmongodb-corelibconnectionpool.js:469:18
    at _combinedTickCallback (internal/process/next_tick.js:73:7)
    at process._tickCallback (internal/process/next_tick.js:104:9)
  name: 'MongoError',
  message: 'E11000 duplicate key error collection: tccdb.groups index: users.user_1 dup key: { : null }',
  driver: true,
  code: 11000,
  index: 0,
  errmsg: 'E11000 duplicate key error collection: tccdb.groups index: users.user_1 dup key: { : null }',
  getOperation: [Function],
  toJSON: [Function],
  toString: [Function] }

var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var UserSchema = new Schema({
    api_id: String,
    user: {
        type: String,
        unique: true
    },
    password: String,
    params: {
        name: String,
        phone: String,
        firebase: String
    }
});

module.exports = mongoose.model('users', UserSchema);

module.exports = function (application) {

    var http = require('http');
    var request = require('request');

    var User = require("../models/UserModel");
    var Message = require("../models/MessageModel");
    var Group = require("../models/GroupModel");

    application.post("/create_group", function (req, res) {
        var data = req.body;

        var create = {};

        var parsed_params = JSON.parse("[]");
        Group.find(function (mongo_error, groups) {
            if (mongo_error) {
                console.log("Erro when connecting to database: " + mongo_error);
                res.send("mongo_error");
            } else {
                var max;
                for (var i = 0; i < groups.length; i++) {
                    if (!max || parseInt(groups[i]["group_id"]) > parseInt(max))
                        max = groups[i]["group_id"];
                }
                console.log("Bigger id is: " + max);
                if (max === undefined) {
                    console.log("There are no groups stored on the database. Group_id will be 1.")
                    create = {
                        title: data.title,
                        group_id: 1,
                        us: [{
                            api_id: data.api_id
                        }],
                        params: parsed_params
                    }

                    var new_group = new Group(create);
                    new_group.save()
                        .then(function (result) {
                            console.log("Group '" + result.title + "' successfully created under id " + result.group_id);
                            res.send(result);
                        })
                        .catch(function (err) {
                            console.log(err);
                            res.send("mongo_error");
                        });
                } else {
                    console.log("There are groups stored on the database. Group_id will be " + ++max)
                    create = {
                        title: data.title,
                        group_id: max,
                        us: [{
                            api_id: data.api_id
                        }],
                        params: parsed_params
                    }

                    var new_group = new Group(create);
                    console.log(new_group);
                    new_group.save()
                        .then(function (result) {
                            console.log("Group '" + result.title + "' successfully created under id " + result.group_id);
                            res.send(result);
                        })
                        .catch(function (err) {
                            console.log(err);
                            res.send("mongo_error");
                        });
                }
            }
        });
    });
}

{
  "title": "Grupo 2",
  "api_id": 3
}