Question

My table has a column “order” that I want initially set to equal the record’s ID.

Can I do this in the migration, or do I need to set it in the create action?

Answer 1

If you already have data, you can’t do this in a create callback action because those items have already been created. However, a create callback would help you for new objects you will create:

after_create :set_order_attribute

def set_order_attribute
  self.update_columns(order: id) if order.nil?
end

You can technically update existing data in a migration as a SQL statement, but I wouldn’t recommend using commands involving the ActiveRecord model as the migration may break sometime down the line as the model changes. To do this in a migration you would have an execute command in the :up method (not sure what behavior you would want for :down) like so:

def up
  table_name = "foo"
  execute "UPDATE #{table_name} SET order = id;"
end

Alternatively you could create a separate rake task to do the update; this would keep your migrations cleaner and give you better control over when you will run the update task.