Migrations

Schema migrations

The Rails Migrations Guide does a very good job at explaining how schema migrations work in Rails.

Not many people know this, but you can pass modifiers when you are generating migrations from the command line.

Whenever possible, write reversible migrations. If a migration can't be reversed, then make sure you raise ActiveRecord::IrreversibleMigration exception.

Must read - take a look at strong_migrations for tips on how to write safer migrations. Please don't hesitate to add the gem to your project, it will help you catch unsafe migrations in development.

Data migrations

Here is where things get a bit complicated. As your project grows and evolves, so does your data. At some point, you might realize you forgot to add a default to a field. Or that you want to change an enumeration. Or manipulate the existing data in your database in any way.

There are two ways of dealing with these problems, and both have its use cases:

  1. Write a rake task
  2. Write a data migration

Your first instinct would be to write a simple rake task. You have access to all your models, and it is easily testable and runnable. You can even delete the file afterwards. The problem with the rake task is that you have to remember to run it. It does not run automatically. Also, if you are writing a big feature and need that change in the middle of your schema migrations, then you have a problem. But if the data migration can be ran after the deployment at any time, and the migration is really really complicated and involves multiple models, then a rake task is the way to go.

Writing a data migration is a bit trickier. You start of by writing a schema migration, but instead of doing add_column or rename_column in your change method, you do a Model.update_all(). Problems start to arise when, after a month of developing a new feature, you introduce some validations that break that migration. Or you remove a method you are using in that migration. Or you remove the model entirely. And you notice the problem only when you try to deploy your application to the server.

Make your data migrations foolproof

Write raw sql queries inside of your data migrations.

Example:

def change
  execute(<<-SQL
    UPDATE users
    SET role =
      CASE role
      WHEN '2' THEN 'developer'
      WHEN '3' THEN 'client'
      END
  SQL
  )
end

When writing raw SQL queries, it's best to use a database client, such as TablePlus or Postico where you can check the syntax and test the query faster. You can also write the query with ActiveRecord and call the to_sql method on the AR query object.

Reversible data migration

If you try to roll back the migration, the example above will, by default, throw ActiveRecord::IrreversibleMigration. If you need to be able to do a rollback, you will need to write your own down method:

def up
  execute(<<-SQL
    UPDATE users
    SET role =
      CASE role
      WHEN '2' THEN 'developer'
      WHEN '3' THEN 'client'
      END
  SQL
  )
end

def down
  execute(<<-SQL
    UPDATE users
    SET role =
      CASE role
      WHEN 'developer' THEN '2'
      WHEN 'client' THEN '3'
      END
  SQL
  )
end