Relations

What are relations? Why do we need them?

Tables are connected
We have more than one table
We can retrieve data easier
We can organise tables better

One to One

person <-> id card person <-> finger print person <-> DNA

person hasOne id id belongTo person

One to Many

person <-> email person <-> bank account person <-> phone number person <-> social media acoounts person <-> address

person hasMany email email belongsTo person

Many to Many

course <-> students user <-> products teachers <-> students user <-> shows person <-> nationality

SET UP

Useful commands

Generate Models & Migrations

npx sequelize-cli model:generate --name user --attributes name:string,email:string,phone:integer,password:string

Generate Seed files

npx sequelize-cli seed:generate --name some-users

Why add relations ?

Not repeating data
Only keep relevant data
to improve performance

Types of relation

One to One:

Person <-> Id card Person <-> Social Security Number Person <-> Drivers License Person <-> Brain

One to Many:

User <-> todoList Person <-> Passports Person <-> phone number

user hasMany images images belongTo user

Many to Many:

person hasMany subjects subjects belongToMany person

Steps to add relation

Step 0: Undo all your migrations and add the Foreign key to the seeds

npx sequelize-cli db:migrate:undo:all

Step 1: Generate a new file to add the relation

npx sequelize-cli migration:generate --name set-up-relations

Step 2: Modify that file to describe the relation

"use strict";

module.exports = {
  up: async (queryInterface, Sequelize) => {
    await queryInterface.addColumn("todoLists", "userId", {
      type: Sequelize.INTEGER,
      references: {
        model: "users",
        key: "id",
      },
      onUpdate: "CASCADE",
      onDelete: "SET NULL",
    });
  },
  down: async (queryInterface, Sequelize) => {
    await queryInterface.removeColumn("todoLists", "userId");
  },
};

Step 3: Migrate and check Postico/DBeaver (here you test the migration files)

npx sequelize-cli db:migrate

Step 4: Write the relations in the models

Step 5: Write queries to test (here you test the model files)

Codaisseur/first-server-59