composed_of with multiple Classes

Help! I'm trying to use composed_of to pull from multiple classes and
can't figure out if it is possible or if there might be a better way
to accomplish what i want.

I created standard_grade_strand_number, which references items in
class Standard. HOWEVER, I don't want to see the the grade_id and
strand_id in class Standard. I want to see the and in their respective classes.

Here's what I have (but not exactly what I want):

class Standard < ActiveRecord::Base
  belongs_to :grade
  belongs_to :strand

  composed_of :standard_grade_strand_number,
              :mapping => [ %w[grade_id grade_id],
                            %w[strand_id strand_id],
                            %w[number number] ]

class StandardGradeStrandNumber < ActiveRecord::Base
  attr_reader :grade_id, :strand_id, :number

  def initialize(grade_id, strand_id, number)
    @grade_id = grade_id
    @strand_id = strand_id
    @number = number

  def to_s
    [ @grade_id, @strand_id, @number ].compact.join("")

Oh, here are the Strand and Grade models:

class Grade < ActiveRecord::Base
  has_many :standards

class Strand < ActiveRecord::Base
  has_many :standards

I've tried putting and in
standard_grade_strand_number but can't make it work.

Do you have any ideas? Any thoughts? Do you see a better way to
accomplish what I want? Thank you in advance!!!