How can I define a BigInt primary key with Rails 2.1 and MySQL?

require 'active_record/connection_adapters/mysql2_adapter'
ActiveRecord::ConnectionAdapters::Mysql2Adapter::NATIVE_DATABASE_TYPES[:primary_key] =
  "BIGINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY".freeze

t.column :product_id, 'BIGINT UNSIGNED'