How to recognize migration direction (up or down) with Rails 3 style migrations (def change)?

Question

I really like the Rails 3 style migrations, i.e. one change method being smart enough to recognize if the migrations is being installed or rolled back, so I don't have to write up and down methods mirroring each other. But I have situation that I need to skip some code when the migration is rolled back (updating counter_cache columns that I'm adding).

I looked at http://guides.rubyonrails.org/migrations.html but the examples at the end of section 5 suffer from the same problem:

class AddFuzzToProduct < ActiveRecord::Migration   class Product < ActiveRecord::Base   end   def change     add_column :products, :fuzz, :string     Product.reset_column_information     Product.all.each { |f| f.update_attributes! :fuzz => 'fuzzy' }   end end 

When this migration is rolled back, the update of fuzz field is unnecessary. Is there a way to prevent it?

I tried looking into Product.column_names but since Rails is smart enough to perform migration in reverse direction, the update is executed before the column is removed. Also, when change method is defined, any up or down methods seem to be ignored. Any other ideas?

Answer 1

Just for future reference, for Rails 4 the best way to do this is to use reversible:

def change   # ...change code...    reversible do |dir|     dir.up do       # ...up-only code...     end   end end 

See http://guides.rubyonrails.org/migrations.html#using-reversible