requirement_graph.rb

 def get_prerequisites(requirement_type, course, requirement_collection)
    course.send(requirement_type).each do |pre|
      requirement_collection << pre.prerequisites
      get_prerequisites(requirement_type, pre, requirement_collection)
    end
  end

  def requirement_graph(requirement_type)
    course_map = [self.send(requirement_type)]
    get_prerequisites(requirement_type, self, course_map)

    return course_map.flatten
  end

  def prerequisites_graph
    requirement_graph(:prerequisites)
  end

  def corequisites_graph
    requirement_graph(:corequisites)
  end