How to reset primary key (??) I wanna set my primary key to translate_str , let it be unique in the table, But I've already create the table and have lots of date in the database.

schema.rb

  create_table "translates", :force => true do |t|
    t.string   "translate_str"
    t.string   "EN"
    t.string   "DE"
    t.string   "ES"
    t.string   "FR"
    t.string   "IT"
    t.string   "JP"
    t.string   "PT"
    t.string   "CN"
    t.string   "TW"
    t.datetime "created_at",    :null => false
    t.datetime "updated_at",    :null => false
  end

translate.rb

class Translate < ActiveRecord::Base
  set_primary_key "translate_str"
  validates_presence_of :CN, :DE, :EN, :ES, :FR, :IT, :JP, :PT, :TW, :translate_str
  attr_accessible :CN, :DE, :EN, :ES, :FR, :IT, :JP, :PT, :TW, :translate_str, :presence => true
end