class Url
  include DataMapper::Resource

  many_to_one :user

  property :link,       String
  property :created_at, DateTime
end




class User
  include AuthenticatedSystem::Model
  include DataMapper::Resource
  include DataMapper::Validate

  one_to_many :urls
....



User[1].urls
 ~
=> #<DataMapper::Associations::OneToMany::Instance:0x336efb8 @dirty_children=[], @parent_resource=#<User:0x336f148 @id=1, @remember_token=nil, @created_at=#<DateTime: 212074459807/86400,0,2299161>, salt"4a3ab75587a708b4951c70b80f3ccd3e300fe6f6", login"jack", loaded_set#<DataMapper::LoadedSet:0x336fcec @key_property_indexes=[0], @repository=#<DataMapper::Repository:0x336ff30 @name=:default, @identity_map=#<DataMapper::IdentityMap:0x336ff08 @cache={User=>{[1]=>#<User:0x336f148 ...}}, @second_level_cache=nil>, adapter#<DataMapper::Adapters::MysqlAdapter:0x339a8ac @name=:default, @resource_naming_convention=DataMapper::NamingConventions::UnderscoredAndPluralized, @field_naming_convention=DataMapper::NamingConventions::Underscored, @uri=#<URI::Generic:0x19cd32a URL:mysql://root@localhost/recollectr_development>, entries[#<User:0x336f148 ...], @properties_with_indexes={#<Property:User:login>=>1, #<Property:User:id=0, #<Property:User:activated_at=6, #<Property:User:crypted_password=3, #<Property:User:created_at=9, #<Property:User:remember_token_expires_at=7, #<Property:User:updated_at=10, #<Property:User:activation_code=5, #<Property:User:salt=4, #<Property:User:remember_token=8, #<Property:User:email=2}, @model=User, @key_properties=[#<Property:User:id>], remember_token_expires_atnil, email"jack.dempsey@gmail.com", crypted_password"a4306161e9d180fde785cecf0bb25baf070f65a2", new_recordfalse, urls_association#<DataMapper::Associations::OneToMany::Instance:0x336efb8 ..., @activated_at=#<DateTime: 106037231483/43200,0,2299161>, parent_associations[#<DataMapper::Associations::OneToMany::Instance:0x336efb8 ...], @updated_at=#<DateTime: 212074459807/86400,0,2299161>, activation_code"4afcae1b4dfa0023d9d6496e48269e70d99277b0", relationship#<DataMapper::Associations::Relationship:0x3367d94 @parent_model_name="User", @name=:user, @repository_name=:default, @parent_properties=nil, @child_properties=nil, @child_model_name="Url", @loader=nil>, children_loader#<Proc:0x01343450@/usr/local/lib/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/associations/relationship.rb:42>
>> User[1].urls.first
 ~
 ~
#<ArgumentError: +key+ must be an Array, and can not be empty>
"SELECT `link`, `created_at`, `user_id` FROM `urls` WHERE (`user_id` = ?)"
ArgumentError: +key+ must be an Array, and can not be empty
        from /usr/local/lib/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/identity_map.rb:19:in `set'
        from /usr/local/lib/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/repository.rb:28:in `identity_map_set'
        from /usr/local/lib/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/loaded_set.rb:45:in `add'
        from /usr/local/lib/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/adapters/data_objects_adapter.rb:172:in `read_set'
        from /usr/local/lib/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/repository.rb:50:in `all'
        from /usr/local/lib/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/associations/one_to_many.rb:36:in `urls_association'
        from /usr/local/lib/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/associations/relationship.rb:43:in `with_parent'
        from /usr/local/lib/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/associations/one_to_many.rb:62:in `call'
        from /usr/local/lib/ruby/gems/1.8/gems/dm-core-0.9.0/lib/data_mapper/associations/one_to_many.rb:62:in `children'
        from (irb):3