gistfile1.rbx

class Group
  include DataMapper::Resource

  property :id,         Integer, :serial => true
  property :name,       String, :length => 256
 
  has n, :states, :through => Resource
end

class State
  include DataMapper::Resource

  property :id,         Integer, :serial => true
  property :name,       String, :length => 256 
  property :code, String

  has n, :groups, :through => Resource
end

-----------------------------------------------------------------
>> g = Group.create(:name => "test")
=> #<Group id=2 name="test">
>> g.states
=> [#<State id=1 name="Puerto Rico" code="PR">]
>> g.states << State.all[1]
=> [#<State id=1 name="Puerto Rico" code="PR">, #<State id=2 name="New Hampshire" code="NH">]
>> g.save
=> true
>> g.states.delete(State.get!(2))
=> #<State id=2 name="New Hampshire" code="NH">
>> g.states
=> [#<State id=1 name="Puerto Rico" code="PR">]
>> g.save  
NoMethodError: undefined method `model' for nil:NilClass from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.5/lib/dm-core/associations/one_to_many.rb:291:in `save_resource'