gistfile1.rb

require 'dm-core'

class A
  include DataMapper::Resource

  property :id, Serial
  has n, :bs, :through => Resource
end

class B
  include DataMapper::Resource

  property :id, Serial
  has n, :as, :through => Resource
end

DataMapper.setup(:default, "sqlite3::memory:")
DataMapper.auto_migrate!

a1 = A.create
a2 = A.create
a3 = A.create
b1 = B.create
b2 = B.create
b3 = B.create
a2.bs = [b1, b2, b3]
a2.save

A.all.each do |a|
  puts [a.id, a.bs].inspect
end

# Output:
# » ruby -rubygems dm_test.rb 
# [1, [#<B @id=1>]]
# [2, [#<B @id=2>]]
# [3, [#<B @id=3>]]