Overview

When writing to a database, it is often important to ensure the data is as expected.

Enter schema-validation with Mongoose.

Mongoose is an object data modeling (ODM) library.

Or in their words,

    "A straight-forward, schema-based solution to model your application data"

Getting Started

• Mongoose replaces the mongodb driver
• Runs mongodb queries under the hood

Make a new directory named mongoose-example

cd mongoose-example
npm init -y
npm install mongoose

Getting Started, cont.

Create a file called mongoose.js In it, write the following code.

example_db in the connection string "mongodb://localhost:27017/example_db" denotes a database name, and can be anything you choose.

const mongoose = require('mongoose')
/* 
pass { useNewUrlParser: true, useUnifiedTopology: true } as a second 
argument to the connect method to avoid deprecation warnings
*/
mongoose.connect('mongodb://localhost:27017/example_db') 
const db = mongoose.connection

db.on('error', console.error.bind(console, 'connection error:'))

Concept: Schemas

A database schema outlines the expected structure of the data that will be inserted into a collection

A database schema can also define methods on the documents being inserted.

• defines what that data should look like, and how it should behave.
• used to create a model
• definitions are based on SchemaTypes

Lab: Schemas

const studentSchema = new mongoose.Schema({
    name: String,
    age: Number,
    hobbies: Array,
    current: Boolean
})

The SchemaType is the 'value' to the right of the :, and is by default a configuration object.

String is supported shorthand for {type:String}. Same goes for Number, Array, Boolean, etc...

The code above defines a simple schema that expects a certain datatype for the given field. A comprehensive list can be found here.

Concept: Model

While the definition of the data's structure is held in the Schema, a Model actually handles the work.

• constructors built using the schema
• enforces definition of schema
• creates a collection based on provided name
• instances of models are documents

Lab: Models

Take our previous schema, studentSchema. Let's create a model from that schema, and call it Student.

const Student = mongoose.model('Student', studentSchema)

const paul = new Student({ name: 'Paul', age: 29, hobbies: ['guitar', 'd&d', 'coding'] })
// call the save() method on a model instance (document) to insert it to the collection 
paul.save()

Or a version with error handling

paul.save((err, paul) => {
    if (err) {
        return console.error(err)
    } else {
        return console.log("document inserted!")
    }
})

Think of Student as a Class with enforcement run by the Schema underneath.

We can create an instance of the Student model like we would any class!

Models and Collections

• creating an instance of a Model makes a document that can be saved to a collection
• that collection is is a lower-case, pluralized version of the Model name
  • Model: Student
  • Collection: students
• the collection can be queried directly with methods on the Model:
  • supports MongoDB query syntax
  • takes a callback for error handling

Seeing it work

Student.find({ name: 'Paul' }, (err, results) => {
    if (err) {
        return console.log(err)
    } else {
        return console.log(results) // => [{name:'Paul, age: 29, ...}]
    }
})

You can also open up Compass and what it does with the collection name. You should see students under example_db, or whatever you ended your connection string with.

Seeing it break

Let's try create an instance of the Student model, and intentionally give it bad data:

let sam = new Student({name:"Samantha", age:"thirty-two", hobbies:["carpentry", "archery"]})
sam.save()
`Student validation failed: age: Cast to Number failed for value "thirty-two" at path "age"`

It thorws an error! It's working!

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