How to Use Migrations and Sync in Sequelize.js

Overview

Migrations are an essential part of modern database management in web development, allowing developers to track and share changes to the database schema. Sequelize.js, a popular ORM for Node.js, comes with robust support for migrations, providing a systematic way to create, modify, and share database schemas. In this tutorial, we’ll cover the basics of setting up migrations and syncing your models with the database using Sequelize.

Setting Up Sequelize

First, ensure that you have Sequelize CLI installed globally:

npm install -g sequelize-cli

Next, initialize Sequelize in your project:

sequelize init

This will create the necessary folders and files, including a config folder that holds your database configuration.

Creating a Migration

To create a new migration file, use the following command:

sequelize migration:create --name create-users

This creates a new file in your ‘migrations’ folder, with boilerplate up and down methods for forward and reverse transactions.

Defining a Migration

Edit the newly created migration file to define the schema for the ‘Users’ table:

module.exports = {
  up: async (queryInterface, Sequelize) => {
    await queryInterface.createTable('Users', {
      id: {
        allowNull: false,
        autoIncrement: true,
        primaryKey: true,
        type: Sequelize.INTEGER
      },
      name: {
        allowNull: false,
        type: Sequelize.STRING
      },
      createdAt: {
        allowNull: false,
        type: Sequelize.DATE
      },
      updatedAt: {
        allowNull: false,
        type: Sequelize.DATE
      }
    });
  },
  down: async (queryInterface, Sequelize) => {
    await queryInterface.dropTable('Users');
  }
};

Running Migrations

To apply migrations to your database, you’ll run:

sequelize db:migrate

Sequelize will execute the ‘up’ method in your migration files to update your database schema.

Model Syncing

Syncing models with the database ensures that the defined models in your application match your database schema. To sync, simply call `sync` on your model:

User.sync(); // Syncs just the User model
sequelize.sync(); // Syncs all models

However, use this with caution in a production environment since it can lead to data loss if the model definitions do not match the migration files.

Advanced Migration Operations

Complex alterations to your database can also be handled with Sequelize migrations. For instance, adding a column:

queryInterface.addColumn(
  'Users', // table name
  'email', // new field name
  {
    type: Sequelize.STRING,
    allowNull: false
  }
);

Or changing a column:

queryInterface.changeColumn(
  'Users',
  'name',
  {
    type: Sequelize.STRING,
    allowNull: true
  }
);

Remember to create and run a new migration for each change to avoid conflicts.

Using Migrations for Rollbacks

If something goes wrong after a migration, you can use the `db:migrate:undo` command to revert the last migration:

sequelize db:migrate:undo

To undo all migrations up to a certain one, you can use:

sequelize db:migrate:undo --to XXXXXXXXXXXXXX-create-users.js

Replace XXXXXXXXXXXXXX with the timestamp in the name of the migration file you wish to revert to.

Conclusion

Migrations are a powerful feature of Sequelize.js that allow for more controlled schema changes and collaborations within development teams. Combined with model syncing, Sequelize provides an elegant solution for managing your application’s data layer. Remember to use these tools wisely, especially in production environments, to keep your data safe and your schema consistent.