Apparently AR doesn't really save the serialized aggregation, although the model seems to be perfectly aware of it.

ar_aggregation_storage.rb

class Prizes < Struct.new(:c1,:c2)
end

class Draw < ActiveRecord::Base
  composed_of :prizes, :mapping => Prizes.members
end

draw = Draw.new

p Draw.column_names.include? "prizes"
# => true

draw.prizes = Prizes.new(1,2)
draw.save!

p draw.prizes
# => #<struct Prizes c1=1, c2=2>

p draw[:prizes]
# => nil