If you're migrating a lot of data, you may feel the need for SQL updates with joins.
Active Record doesn't have an abstraction for this, but you can run the SQL manually.
Note that this is for Postgres. The syntax likely differs for other database engines.
It took me a while to figure out the syntax the first time I did it, though, so here it is for reference.
In this contrived example, an order belongs to an item which belongs to a company, and you want to set every order's
company_code from its item's company's
class MyMigration < ActiveRecord::Migration
SET company_code = companies.code
FROM items, companies
orders.item_id = items.id AND
items.company_id = companies.id