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Introduction to Sequelize Migrations

What are Migrations

Just like how we use Git to version control source code, we use migrations to manage the state of our database schemas.

I'm not really sure what that means...

Imagine you're working on project with another developer, and you're both tasked with creating a specific part of an event planning application. Let's say you are in charge of creating the Users and your friend is going to create the Events.

Let's say you and your friend divided the work in a way so that neither of you will have to to use each other's code to finish your tasks. While you're working on your part of the application, you only really need to touch the Users table when you are working with the database.

Before you begin

Make sure that the project you are in is a node project (it has a package.json) and you have already installed and initialized sequelize (npm install --save sequelize, sequelize init). Also make sure that your config.json file has the correct credentials to connect to your database.

Creating models and migrations

Using sequelize-cli you can easily create and manage your models and migrations. It has a useful command called model:create, which will generate 2 files for you: a model file and a corresponding migration file.

It has a couple handy options so that you can create your schemas from the command line:

Example Usage

sequelize model:create --name User --attributes first_name:string,last_name:string,bio:text

Important: There are no spaces between attributes!

Notice the --name and --attributes flags. these let us tell the program that we want to create a new model called User, that has 3 properties and their types:

  • first_name, type: string
  • last_name, type: string
  • bio, type: text

For a list of all the valid data types Sequelize has to offer, see: http://docs.sequelizejs.com/en/latest/api/datatypes/

This will generate 2 files for you: models/user.js

'use strict';
module.exports = function(sequelize, DataTypes) {
  var User = sequelize.define('User', {
    first_name: DataTypes.STRING,
    last_name: DataTypes.STRING,
    bio: DataTypes.STRING
  }, {
    classMethods: {
      associate: function(models) {
        // associations can be defined here
      }
    }
  });
  return User;
};

migrations/{timestamp}-create-user.js

'use strict';
module.exports = {
  up: function(queryInterface, Sequelize) {
    return queryInterface.createTable('Users', {
      id: {
        allowNull: false,
        autoIncrement: true,
        primaryKey: true,
        type: Sequelize.INTEGER
      },
      first_name: {
        type: Sequelize.STRING
      },
      last_name: {
        type: Sequelize.STRING
      },
      bio: {
        type: Sequelize.TEXT
      },
      createdAt: {
        allowNull: false,
        type: Sequelize.DATE
      },
      updatedAt: {
        allowNull: false,
        type: Sequelize.DATE
      }
    });
  },
  down: function(queryInterface, Sequelize) {
    return queryInterface.dropTable('Users');
  }
};

Let's take a look at the migration file. If you look carefully at the name of the file, it will have the timestamp of when the migration file was created. This is on purpose so that sequelize can run the older migration files first, and then the newer ones that build on top of them. This is important later when changes need to be made to the database.

Also, it's guaranteed that your models and your migrations match. The migration files keep track of the state of the database schemas, and should always be created when changes to your models are made. Always commit coresponding migration files when changes to your models are made.

For a list of migration functions, see: http://sequelize.readthedocs.org/en/latest/docs/migrations/

Woops! Forgot a something...

Let's pretend you have jammed out 50 new commits while working with your newly created Users table, merged in your friends Events, and realize that you forgot to add in an email column to the table! Don't worry, this is not a problem.

sequelize migration:create --name add-email-to-user

This time you run the command, it only generates a migration file for you. When you open it, you notice it's pretty bare!

migrations/-add-email-to-user.js

'use strict';

module.exports = {
  up: function (queryInterface, Sequelize) {
    /*
      Add altering commands here.
      Return a promise to correctly handle asynchronicity.

      Example:
      return queryInterface.createTable('users', { id: Sequelize.INTEGER });
    */
  },

  down: function (queryInterface, Sequelize) {
    /*
      Add reverting commands here.
      Return a promise to correctly handle asynchronicity.

      Example:
      return queryInterface.dropTable('users');
    */
  }
};

But, because you're awesome and read the documentation, you know sequelize migrations has an addColumn function that will fix the problem for you!

After filling in the up and down functions, your migration file looks like this:

'use strict';

module.exports = {
  up: function (queryInterface, Sequelize) {
    queryInterface.addColumn( 'Users', 'email', Sequelize.STRING );
  },

  down: function (queryInterface, Sequelize) {
    queryInterface.removeColumn( 'Users', 'email' );
  }
};

It's important to always have both up and down methods in your migration script. Every time you run sequelize db:migrate it will call the up method. And if you ever want to revert backwards, you can call sequelize db:migrate:undo which will call the down method. They should always be opposite actions, this allows you to safely run migrations and know that the state of the database will be intact.

Not so fast! Update your model too!

Now that you fixed your migrations, you need to update your model to reflect the changes.

models/user.js

'use strict';
module.exports = function(sequelize, DataTypes) {
  var User = sequelize.define('User', {
    first_name: DataTypes.STRING,
    last_name: DataTypes.STRING,
    bio: DataTypes.TEXT,
    email: {
      type: DataTypes.STRING,
      validate: {
        isEmail: true
      }
    }
  }, {
    classMethods: {
      associate: function(models) {
        // associations can be defined here
      }
    }
  });
  return User;
};

Secret read unlocked! Validations: http://docs.sequelizejs.com/en/latest/docs/models-definition/#validations

Now if you run migrations, it will update your table to have emails as well!

sequelize_express=# SELECT * FROM "Users";
 id | first_name | last_name | bio | createdAt | updatedAt | email 
----+------------+-----------+-----+-----------+-----------+-------
(0 rows)

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