flouri.sh

Using ActiveRecord can be tricky, here's how

Data migrations are a major headache in every rails project I've worked on. Developers typically write straight SQL migrations which take much longer to create and test, or they use ActiveRecord and run into problems. I recent wrote a data migration which took 7 existing tables and compressed them into 4. Some of the new 4 had the same names as the existing models, so I finally figured out how to do this safely with ActiveRecord. Here's my advice:

• When changing the schema of an existing set of tables, create new tables and then rename them.
• Include active record fragments inside the migration class at the top of your migrations.
• Put your data migrations inside transactions.
• Make your data migrations completely reversible (which is a lot easier when you follow the first rule)

Here's an example of an ActiveRecord class fragment at the top of a migration:

class MarkPrimaryBits < ActiveRecord::Migration
  class LessonVersion < ActiveRecord::Base
    belongs_to :lesson
  end

  class Lesson < ActiveRecord::Base
    has_many :lesson_versions
  end
 
  def self.up
    ...

By putting the Lesson and LessonVersion classes at the top of the migration, I'm allowing myself to use those classes inside my migration and be completely independent of any changes made to the real model from then on (including the deletion of the class itself). Furthermore, I can have another migration which uses those same class names with completely different meanings and they won't conflict with each other.