aggregation: use composed_of with multiple classes (tables)

Does anyone know how to use composed_of to pull data from multiple
classes (tables)? I know how to use composed_of to aggregate columns
within the same table but am running in to trouble pulling from two

The example below works but I don't want to see standard.strand_id, I
want to see (which is in the Strand table, not the
Standard table).

class Standard < ActiveRecord::Base
  composed_of :full_standard,
              :mapping =>
                  %w[grade grade],
                  %w[strand_id strand_id]
                  %w[number number]

class FullStandard < ActiveRecord::Base
  attr_reader :grade, :strand_id, :number

  def initialize(grade, strand, number)
    @grade = grade
    @strand_id = strand_id
    @number = number

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

Again, this works, BUT I don't want to see the strand_id, I want to
see (a column in the Strand table--not the Standard table)

Also, here are the table relationships:

class Standard
belongs_to :strand

class Strand
has_many :standards

Thank you!!!

Is anyone able to share some insight regarding composed_of pulling
from two tables?