fun_with_dm_associations.rb

require 'rubygems'
require 'datamapper'
DataMapper::Logger.new(STDOUT, :debug)
DataMapper.setup(:default, 'sqlite3::memory:')

class A
  include DataMapper::Resource
  property :id,Serial
  has n, :bs
end

class B
  include DataMapper::Resource
  property :id,Serial
  belongs_to :aa,:class_name=>'A',:child_key=>[:aa_id]
  def z
    self.aa = A.new
  end
  before :valid?, :z
end
DataMapper.auto_migrate!
x = B.new
x.aa = A.new
x.save
p B.first.aa.bs
#=> []