polymorphic.rb

class Teacher
  has_many :course_memberships, :as => :takeable
  has_many :courses, :through => :course_memberships
end

class Student
  has_many :course_memberships, :as => :takable
  has_many :courses, :through => :course_memberships
end

class CourseMembership 
  belongs_to :course 
  belongs_to :takable, :polymorphic => true
end

class Course                       #note this model is not the polymorhic end
  has_many :course_memberships
  has_many :teachers, :through => :course_memberships
  has_many :students, :through => :course_memberships
end