Automatic Rails migrations
To ensure PlanetScale works well with a traditional Rails development process, we implemented the ability to automatically copy Rails migration metadata as part of our deployment process.
If you are using PlanetScale with a Rails application, go to your database's Settings page in the web app and enable "Automatically copy migration data." Select "Rails/Phoenix" as the migration framework. When enabled, this setting updates the schema_migrations table each time you branch with the latest migration. If disabled, running rake db:migrate will try to run all migrations every time, instead of only the latest one.
In this tutorial, you're going to learn how Rails migrations work with the PlanetScale branching and deployment workflows.
Migration tracking works with any migration tool, not just Rails. For other frameworks, specify the migration table name on your database's Settings page.
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
- Started a new Rails app named
blogwith a migration creating a
- 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
- Rails queries the
schema_migrationstable to see which migrations have and haven't been run.
- Any migration that doesn’t appear in the
schema_migrationstable is considered pending and is executed by this task.
When you merge a deploy request in PlanetScale, the schema_migrations table in main is automatically updated with the migration data from your branch.
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
- Create an
add-posts-tabledevelopment branch from
mainin your database blog:
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:
Connect to the new branch:
Query the migration table:
- 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:
config/database.yml to connect through the proxy:
- Create the second Rails migration and call it
Find the new migration file in
db/migrate and add a few details for the new Posts table:
- Run the CreatePosts migration:
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
- Open a deploy request for your
add-posts-tablebranch, and deploy your changes to
You can complete the deploy request either in the web app or with the
pscale deploy-request command.
To create the deploy request, PlanetScale looks at the differences between the schemas of
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
add-posts-table to the
schema_migrations table in
- Verify the changes in your
pscale shell for
main you can verify that the changes from
add-posts-table were deployed successfully.
In this tutorial, we learned how to use the PlanetScale deployment process with the Rails migration workflow.
Learn more about how PlanetScale allows you to make schema changes to your production databases without downtime or locking tables.