Question

I’m changing a relationship in my database from has_many to a has_many :through. So right now I have:

class Brand < Ar::Base
  has_many :products
end

class Product < AR::Base
  belongs_to :brand
end

and I’m going to add a join table.

But of course I need to update the database with data after that. I have seen that it is not good practice to do this in the confines of the migration. Where is the best place to perform this, knowing that I have to then run another migration after the data update is complete (i.e. removing the original brand_id column from the products table)?

Answer 1

Unless I misunderstand your question, the migration is the place to make that transformation. The purpose of the migration is to change your schema and migrate existing data to use the schema. Migrations capture the temporal aspect of layering schema changes, so that you can go forward and backward in time without leaving data in an inconsistent state. If you were to migrate your rows anywhere else, you have no guarantee that when that code runs, the schema is as it was when you wrote your migration code.

I believe that you will find support for my position in the examples on the Active Record Migrations api documentation. You might be confusing migrations with populating seed data (rake db:seed) which is handled in db/seeds.rb.