Be Careful With Your Database Migration

In the early stage as a Start-Up, we do many experiments to prove our assumption to reach product market fit. Updating database schema could be a daily or weekly task, and it should be done correctly. Especially when we have reached product market fit, many users are using our product.

Example scenario: We do this when deleting a column in Rails

class RemoveColumn < ActiveRecord::Migration
 def change
  remove_column :users, :promo_code
 end
end

When we deploy the code into production and execute the migration command (rails db:migrate), any running servers will crash if there is a process that reads the promo_code column. The safe way to do that is to deploy a new version to ignore the promo_code column.

class User < ApplicationRecord
 self.ignored_columns = ["promo_code"]
end

Once the deployment is done, then delete the column.

class RemoveSomeColumnFromUsers < ActiveRecord::Migration
 def change
  remove_column :users, :promo_code
 end
end

Fortunately, there is a RubyGem to prevent bad things happened. It’s called Strong Migrations