Rails - Nov 2005 - How to populate join tables?

Given: 

class CseModule < ActiveRecord::Base
  has_and_belongs_to_many :students
  belongs_to :staff
end

class Student < ActiveRecord::Base
  has_and_belongs_to_many :cse_modules
end

Why doesn''t

      # This next line should sort out the join table.
      @student_modules.each do |student, modules|
        the_student = Student.find_pnumber(student.pnumber)
        modules.each do |cse_module|
          the_cse_module = CseModule.find_aos_code(cse_module.aos_code)
          the_cse_module.students << the_student
        end
      end

leave any data in the join table cse_modules_students ?
According to my understanding of pages 240-241 of Agile Web
development it should.  I don''t get any exceptions thrown.  I
didn''t
get errors about nonexistent tables being accessed.

        Thank you,
        Hugh