has_many_through.rb

class Teacher < ActiveRecord::Base
  has_many :courses
  has_many :lightbulbs, :through => "courses"
end

class Course < ActiveRecord::Base
  has_many :lightbulbs
end

class Lightbulb < ActiveRecord::Base
end

teacher = Teacher.find 1
teacher.courses

#I thought the relationships were more complicated than this, but it maps to the example you send and hopefully illustrates the point