Automatic Rails migrations

Introduction

In this tutorial, you're going to learn how Rails migrations work with the PlanetScale branching and deployment workflows.

Prerequisites

Follow the Connect a Rails app tutorial first. By the end, you will have:

• Installed the PlanetScale CLI, Ruby, and the Rails gem
• Created a PlanetScale database named blog
• Started a new Rails app named blog with a migration creating a Users table
• Run the first Rails migration

A quick introduction to Rails migrations

Rails tracks an application's migrations in an internal table called schema_migrations. At a high level, running rake db:migrate does the following:

• Rails looks at all of the migration files in your db/migrate directory.
• Rails queries the schema_migrations table to see which migrations have and haven't been run.
• Any migration that doesn’t appear in the schema_migrations table is considered pending and is executed by this task.

Execute a Rails migration on PlanetScale

Rails migrations follow the PlanetScale non-blocking schema change workflow. First, the migration is applied to a development branch, and then the development branch is merged into the main production branch.

Let's add another table to your existing blog schema:

1. Create an add-posts-table development branch from main in your database blog:

pscale branch create blog add-posts-table

When the branch is ready, you can verify that the schema_migrations table is up-to-date with main by checking for the timestamp of your Create Users migration file. Your migration will have a different timestamp than the one shown here.

Check the timestamp in your codebase:

ls db/migrate
20211014210422_create_users.rb

Connect to the new branch:

pscale shell blog add-posts-table

Query the migration table:

blog/add-posts-table> select * from schema_migrations;
+----------------+
| version        |
+----------------+
| 20211014210422 |
+----------------+

1. Connect your development environment to the new branch:

One way to do this is to create a new password for the add-posts-table branch and update config/database.yml with the new username, password, and host. Another is to use pscale connect to establish a secure connection on a local port. Since the add-posts-table branch won't be needed after the migration, let's use the pscale connect proxy.

In a separate terminal, establish the connection:

pscale connect blog add-posts-table --port 3309

Then, update config/database.yml to connect through the proxy:

development:
  <<: *default
  adapter: mysql2
  database: blog
  host: 127.0.0.1
  port: 3309

1. Create the second Rails migration and call it CreatePosts:

rails generate migration CreatePosts

Find the new migration file in db/migrate and add a few details for the new Posts table:

class CreatePosts < ActiveRecord::Migration[7.0]
  def change
    create_table :posts do |t|
      t.string :title
      t.text :content
      t.bool :published
      t.references :user
      t.timestamps
    end
  end
end

1. Run the CreatePosts migration:

rake db:migrate

This command runs the new migration against your add-posts-table development branch.

At this point, Rails creates the posts table and inserts another timestamp into the schema_migrations table on your development branch.

You can verify the change in schema_migrations yourself:

blog/add-posts-table> select * from schema_migrations;
+----------------+
| version        |
+----------------+
| 20211014210422 |
| 20220224221753 |
+----------------+

1. Open a deploy request for your add-posts-table branch, and deploy your changes to main.

You can complete the deploy request either in the web app or with the pscale deploy-request command.

pscale deploy-request create blog add-posts-table

pscale deploy-request deploy blog 1

To create the deploy request, PlanetScale looks at the differences between the schemas of main and add-posts-table and plans a create table statement to add the new table to main. When you deploy, PlanetScale runs that create table statement and copies the second row from schema_migrations in add-posts-table to the schema_migrations table in main.`

1. Verify the changes in your main branch:

In a pscale shell for main you can verify that the changes from add-posts-table were deployed successfully.

pscale shell blog main

blog/|⚠ main ⚠|> show tables;
+----------------------+
| Tables_in_blog       |
+----------------------+
| posts                |
| schema_migrations    |
| users                |
+----------------------+

blog/|⚠ main ⚠|> select * from schema_migrations;
+----------------+
| version        |
+----------------+
| 20220223232425 |
| 20220224221753 |
+----------------+

Summary

In this tutorial, we learned how to use the PlanetScale deployment process with the Rails migration workflow.

What's next?

Learn more about how PlanetScale allows you to make schema changes to your production databases without downtime or locking tables.