Example of a (slightly hacky) concurrent migration for Rails 3.2 to add an index to a huge table without locking the table in a transaction.

add_index_on_client_application_id_for_oauth_tokens.rb

class AddIndexOnClientApplicationIdForOauthTokens < ActiveRecord::Migration
  def ddl_transaction(&block)
    # hack because AR 3.x doesn't support the `disable_ddl_transaction!`
    # method that AR 4.x introduced yet.
    block.call # do not start a transaction
  end

  def change
    # using raw SQL because ActiveRecord 3.x doesn't support concurrent
    # migrations yet, in AR 4 this would be:
    #
    # add_index :oauth_tokens, :client_application_id, algorithm: :concurrently
    #
    execute "END" # manually end the current transaction
    execute "CREATE INDEX CONCURRENTLY index_oauth_tokens_on_client_application_id ON oauth_tokens(client_application_id)"
    execute "BEGIN" # manually start a new transaction
  end
end

I was incorrect -- the actual issue in Rails 3 is that ddl_transaction moved into ActiveRecord::Migrator.