##the assertion fails saying the length is zero
def test_position
    subject_one =courses(:one).course_subjects.create!(:position => courses(:one).subjects.length)
    subject_two =courses(:one).course_subjects.create!(:position => courses(:one).subjects.length)
    subject_three =courses(:one).course_subjects.create!(:position => courses(:one).subjects.length)
    subject_one.subject = subjects(:one)
    subject_two.subject = subjects(:two)
    subject_three.subject = subjects(:three)
    subject_one.subject.save
    assert_equal courses(:one).subjects.length, 3
 end
##a course
class Course < ActiveRecord::Base
  belongs_to :author, :class_name=>'User'
  has_many :course_subjects, :order => :position, :before_add => Proc.new {| course, subject| subject.position = course.subjects.length}
  has_many :subjects, :through => :course_subjects
end
##a course subject
class CourseSubject < ActiveRecord::Base
  belongs_to :course
  belongs_to :subject
end
##a subject
class Subject < ActiveRecord::Base
  belongs_to :subject
  has_many :course_subjects, :order => :position
  has_many :courses, :through => :course_subjects
end