Help With id

Hello. I have my rb file:

class CreateDepartamentos < ActiveRecord::Migration
  def self.up
    create_table :departamentos, :id=>null do |t|
      t.string :cvedept, :limit=>10, :null=>false
      t.string :desdept, :limit=>20, :null=>false


  def self.down
    drop_table :departamentos

but when i try with a new record i get the next error:

Unown columno "" in table "departamentos".

but i dont have the field "id" in my table.

Help please!.

you need to tell rails what to use as your primary key (via the
set_primary_key function)