In Rails 3.1 and newer, when you write a migration by hand, you can (usually) just define a change method instead of an up and a down method.

This StackOverflow answer from user Readonly demonstrates the use of change nicely:

class RenameOldTableToNewTable< ActiveRecord:Migration
  def self.up
    rename_table :old_table_name, :new_table_name
  end
  def self.down
    rename_table :new_table_name, :old_table_name
  end
end

becomes

class RenameOldTableToNewTable< ActiveRecord:Migration
  def change
     rename_table :old_table_name, :new_table_name
  end
end