How can i use MongoDB with Cloud Functions for Firebase?

I want to use Cloud Functions for Firebase and MongoDB. The problem is I don't know how to connect my Mongo database with Cloud Functions. My database is deployed at matlab. I made this schema:

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

var patientSchema = new Schema({

    name: {
        type: String,
        required : true,
    },

    disease:{
        type: String,
        required : true, 
    },

    medication_provided: {
        type: String,
        required : true,
    },

    date : {
        type : Date,
    }
})

const patient = mongoose.model('patientInfo', patientSchema)
module.exports = patient

Then I require my schema in project index.js file, and export a function called getAllPatient.

const patient = require('../Patient')
const functions = require('firebase-functions');
const mongoose = require('mongoose')

mongoose.connect('mongodb://patient:patient123@ds139869.mlab.com:39869/patient',{useMongoClient: true})
exports.getAllPatient = functions.https.onRequest((request, response) => {
    patient.find({}).then((data) => {
        response.send(data)
    })
})

but gives me an error that "Error: could not handle the request"