Datamapper DataMapper

When I do:

>> @course= Course.new(:title=>"asd")
=> #<Course id=nil title="asd">
>> @task = Task.new
=> #<Task id=nil title=nil deadline=nil>
>> @task.course = @course
=> #<Course id=nil title="asd">
>> @task.save
=> true

course as well as task is saved in the database, but course_id field in tasks is empty. I have to save the course first then do the @task.course = @course so course_id is filled. When I save @course before saving @task, @task.course points to the saved @course with filled id in it, but when I do @task.save tasks.course_id isn't filled.

My Models:

## Model Course
class Course
  include DataMapper::Resource

  property :id, Serial
  property :title, String

  has n, :tasks
end

## Model Task
class Task
  include DataMapper::Resource

  property :id, Serial
  property :title, String
  property :deadline, DateTime

  belongs_to :course
end