README.md

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In this lesson, we will learn to define associations(a.k.a. relationships) between two Sequelize models.

In our application, to show the To-Dos of a logged-in user, we have to define the relationship between the Todo and User model. The relation would be user has many todos, right? Let's define that.

Script

In this video, we will learn about the steps associated in defining association between two Sequelize models.

1. First, we have to update our Todos table to add a new foreign key userId, which will reference to the Users table.
2. Then, we have to define the has many and belongs to relation in both User and Todo model, respectively.

So, let's get started.

Creating a new Migration

To add the userId column in Todos table, we will create a new migration file in the terminal.

npx sequelize-cli migration:create --name add-user-id-in-todos

In VS Code, let's open the migration file. It already comes with a default template, where we have to write just few lines of code to add a new column.

'use strict';

module.exports = {
  async up (queryInterface, Sequelize) {
    await queryInterface.addColumn('Todos', 'userId', {
      type: Sequelize.DataTypes.INTEGER
    })
    await queryInterface.addConstraint('Todos', {
      fields: ['userId'],
      type: 'foreign key',
      references: {
        table: 'Users',
        field: 'id'
      },
      onDelete: 'cascade',
      onUpdate: 'cascade'
    })
  },

  async down (queryInterface, Sequelize) {
    await queryInterface.removeColumn('Todos', 'userId');
  }
};

Here, we've used addConstraint from the Sequelize queryInterface to define that, userId is a foreign key in Todos table, which actually refers to the id column of Users table.

So, we can run our migration:

npx sequelize-cli db:migrate

The command executed successfully, great! Let's check the database once.

Open PGAdmin to show the db structure.

Defining Associations

Now, open the User model, which you'll find in the models folder. There we will define the association details:

  class User extends Model {
    /**
     * Helper method for defining associations.
     * This method is not a part of Sequelize lifecycle.
     * The `models/index` file will call this method automatically.
     */
    static associate(models) {
      // define association here
      User.hasMany(models.Todo, {
        foreignKey: 'userId',
      })
    }
  }

Here, we've defined that User has many Todo and the foreign key is userId (in the Todos table).

Similarly, open the Todo model:

  class Todo extends Model {
    /**
     * Helper method for defining associations.
     * This method is not a part of Sequelize lifecycle.
     * The `models/index` file will call this method automatically.
     */
    static associate(models) {
      // define association here
      Todo.belongsTo(models.User, {
        foreignKey: 'userId',
        onDelete: 'CASCADE'
      })
    }
  }

Here, we've defined that, every Todo belongs to an User.

That's it, in this lesson, we've successfully defined all associations. See you in the next video.