ORM : Sequelize

1. Sequelize only provides ORM, to interact with a database, a database driver is still needed, like mysql2.
2. In sequelize, database tables are referred as models. A model is an abstraction that represents a table in the database.
3. To create a database connection, put the database name, username and password in an environment variable.

require('dotenv').config()
const {Sequelize} = require('sequelize')
sequelize = new Sequelize(
        process.env.DB_NAME,
        process.env.DB_USERNAME,
        process.env.DB_PASSWORD,
        {
            host: process.env.DB_HOST,
            dialect: 'mysql'
        }
    );

4 To create an instance of Model, use sequelize.define() or class User extends Model. The instance can have method.

const { Model, DataTypes } = require('sequelize');
class User extends Model {
    async checkPassword(password) {
        try {
            const matchPassword = await bcrypt.compare(password, this.password);
            return matchPassword;
        } catch (err) { console.error(err); }
    }
}

5 By default, sequelize add createAt and updateAt columns, to prevent it use timestamps: false. freezeTableName: true will make the table name same with model name.

6 sequelize.sync({force: true}) will synchronize the model to database. It creates a new table and drops the existing one first.

7 Sequelize hooks are functions called before or after sequelize calls are executed. The code below will hash the password everytime before a new user is created and saved.

hooks: {
    beforeCreate: async (userData) => {
        const salt = await bcrypt.genSalt(10);
        const hashedPassword = await bcrypt.hash(userData.password, salt);
        userData.password = hashedPassword;
    },
},

8 Populate the database: model.create() or model.bulkCreate(). model.update() to update the data. model.destroy() to delete the data.

9 Three common types between tables: one to one, one to many and many to many.

• One to One

User.hasOne(Passport)
Passport.belongsTo(User)

• One to many

User.hasMany(Post)
Post.belongsTo(User)

• Many to many: The studentClass table will have three columns: id, student_id and class_id

Student.belongsToMany(Class, {through: studentClass, foreighKey: student_id})
Class.belongsToMany(Student, {through: studentClass, foreignKey: class_id})

10 model.findAll() can find all the data in the mobel. It can be used with where clause and operators

 const postData = await Post.findAll(
    {
        where: { user_id: req.user.id },
        include: { model: User }
    });

11 model.findOne() will return the one matching data. It can include the data from other models.

 const postData = await Post.findOne({
    where: { id: req.params.id },
        include: [
            { model: User },
            { 
                model: Comment,
                include: [User] // here is a join of comment and user table
            }]
    });