Relations

What are relations? Why do we need them?

Why add relations?

Types of relations

One to One:

User <-> IdCard Person <-> Birth Certificate Person <-> Toothbrush Person <-> DNA Person <-> Spouse

One to Many:

Person <-> Books Person <-> Bank account Person <-> Phone numbers

person.hasMany(models.book) boook, belongTo(models.person)

Many to Many:

students <-> classes products <-> user

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)