pre.save() not working

'use strict';

var mongoose = require('mongoose');

mongoose.connect('mongodb://localhost:27017/sandbox');

var db = mongoose.connection;
db.on('error', (error) =>{
    console.log('connection error: ', error);
});

db.once('open', () => {
    console.log('database connection was successful');
    // Database codes goes here...

    var schema = mongoose.Schema;

    // Creating default values for mongoDB with mongoose
    var animalSchema = new schema({
        type:  {type: String, default: 'goldfish'},
        color: {type: String, default: 'golden'},
        size: String,
        mass:  {type: Number, default: 0.007},
        name:  {type: String, default: 'goldy'}
    });

    animalSchema.pre('save', (next) => {
        console.log(this);
        if(this.mass >= 100){
            this.size ='big';
        } else if(this.mass >= 5 && this.mass < 100){
            this.size = 'medium';
        } else {
            this.size = 'small';
        }
        next();
    });

    var Animal = mongoose.model('Animal', animalSchema);

    var elephant = new Animal({
       type: 'elephant',
       color: 'gray',
       mass: 250,
       name: 'lawrence'
    });

    var animal = new Animal({}); // create goldfish

    var whale = new Animal({
        type: 'whale',
        mass: 1500,
        name: 'jessie'
    });

    var animalData = [
        {
            type: 'mouse',
            color: 'gray',
            mass: 0.032,
            name: 'marvin'
        },
        {
            type: 'nutria',
            color: 'brown',
            mass: 6.35,
            name: 'gretchen'
        },
        elephant,
        animal,
        whale
    ];

    Animal.remove({}, (error) => {
        if(error){
            console.error(error);
        }
        Animal.create(animalData, (error, animals) => {
            if(error){
                console.error(error);
            }
            Animal.find({}, (error, animals) => {
                animals.forEach((animal) => {
                    console.log(`${animal.name} the ${animal.color} ${animal.type} and is a ${animal.size}-sized animal.`)
                    });
                db.close(() => {
                    console.log('database closed');
                });
            });
        });
    });
});