Problem with composite_primary_key Gem in Rails

I'm using the composite primary keys gem from drnic and I've got a problem with it: If I want to create a CourseOrder (with has a cpk) with the following command in my tests: course_order = CourseOrder.new(:daily_order => daily_orders(:daily_order_one_of_today_for_hans),:course => courses(:non_veg_2), :finished => false, :ordered_at => Time.now) I'll get the following error:

ActiveRecord::StatementInvalid: PGError: ERROR: null value in column "course_iddaily_order_id" violates not-null constraint : INSERT INTO "course_orders" ("course_iddaily_order_id", "course_id", "ordered_at", "finished", "daily_order_id") VALUES (NULL, 489519433, '2011-03-01 10:19:27.169397', 'f', 594369222) RETURNING "course_iddaily_order_id"

I think that there is a course_iddaily_order_id is wrong, isn't it?

My Migration file looks like this:

class AddCourseOrder < ActiveRecord::Migration
  def self.up
    create_table :course_orders, :primary_key => [:course_id, :daily_order_id] do |table|
      table.integer :course_id
      table.integer :daily_order_id
      table.datetime :ordered_at, :null => false
      table.boolean :finished, :default => false, :null => false
    end
  end

  def self.down
    drop_table :course_orders
  end
end

The resulting schema part looks like this:

  create_table "course_orders", :primary_key => "course_iddaily_order_id", :force => true do |t|
    t.integer  "course_id",                         :null => false
    t.integer  "daily_order_id",                    :null => false
    t.datetime "ordered_at",                        :n开发者_如何学编程ull => false
    t.boolean  "finished",       :default => false, :null => false
 end

My Model looks like this:

class CourseOrder < ActiveRecord::Base
  set_primary_keys :course_id, :daily_order_id
  belongs_to :course 
  belongs_to :daily_order
end

I don't know whats wrong here, can you help?

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I guess the create_table method doesn't takes an array for :primary_key option. I guess you can omit setting it.