dm_self_referential_friends.rb

require 'dm-core'
require 'spec'

DataMapper::Logger.new(STDOUT, :debug)
DataMapper.setup(:default, "mysql://localhost/dm_friends")

class User
  include DataMapper::Resource
  
  property :id, Serial
  property :name, String
  
  has n, :friendships, :model => "User::Friendship"
  has n, :friends, :through => :friendships, :model => "User", :child_key => [:friend_id]
  
  class Friendship
    include DataMapper::Resource
    
    property :user_id, Integer, :key => true
    property :friend_id, Integer, :key => true
    
    belongs_to :user, :child_key => [:user_id]
    belongs_to :friend, :model => "User", :child_key => [:friend_id]
  end
end

DataMapper.auto_migrate!

describe User do
  it "can have friends" do
    @ted = User.create(:name=> "Ted")
    @dan = User.create(:name=> "Dan")
    User::Friendship.create(:user_id => @ted.id, :friend_id => @dan.id)
    @ted.friendships.should_not be_empty
    @ted.friends.should_not be_empty
  end
end

gistfile2.txt

umbra:dm-specs ted$ irb
>> require 'dm_self_referential_friends'
 ~ (0.000084) SET sql_auto_is_null = 0
 ~ (0.000073) SET SESSION sql_mode = 'ANSI,NO_AUTO_VALUE_ON_ZERO,NO_DIR_IN_CREATE,NO_ENGINE_SUBSTITUTION,NO_UNSIGNED_SUBTRACTION,TRADITIONAL'
 ~ (0.002173) DROP TABLE IF EXISTS "user_friendships"
 ~ (0.001259) DROP TABLE IF EXISTS "users"
 ~ (0.000443) SHOW TABLES LIKE 'user_friendships'
 ~ (0.000066) SET sql_auto_is_null = 0
 ~ (0.000072) SET SESSION sql_mode = 'ANSI,NO_AUTO_VALUE_ON_ZERO,NO_DIR_IN_CREATE,NO_ENGINE_SUBSTITUTION,NO_UNSIGNED_SUBTRACTION,TRADITIONAL'
 ~ (0.000703) SHOW VARIABLES LIKE 'character_set_connection'
 ~ (0.000607) SHOW VARIABLES LIKE 'collation_connection'
 ~ (0.342712) CREATE TABLE "user_friendships" ("user_id" INTEGER NOT NULL, "friend_id" INTEGER NOT NULL, PRIMARY KEY("user_id", "friend_id")) ENGINE = InnoDB CHARACTER SET utf8 COLLATE utf8_general_ci
 ~ (0.000361) SHOW TABLES LIKE 'users'
 ~ (0.098797) CREATE TABLE "users" ("id" INTEGER NOT NULL AUTO_INCREMENT, "name" VARCHAR(50), PRIMARY KEY("id")) ENGINE = InnoDB CHARACTER SET utf8 COLLATE utf8_general_ci
=> true
>>     @ted = User.create(:name=> "Ted")
 ~ (0.032618) INSERT INTO "users" ("name") VALUES ('Ted')
=> #<User @id=1 @name="Ted">
>>     @dan = User.create(:name=> "Dan")
 ~ (0.001171) INSERT INTO "users" ("name") VALUES ('Dan')
=> #<User @id=2 @name="Dan">
>>     User::Friendship.create(:user_id => @ted.id, :friend_id => @dan.id)
 ~ (0.001395) INSERT INTO "user_friendships" ("user_id", "friend_id") VALUES (1, 2)
=> #<User::Friendship @user_id=1 @friend_id=2>
>>     @ted.friendships
 ~ (0.000275) SELECT "user_id", "friend_id" FROM "user_friendships" WHERE "user_id" = 1
=> [#<User::Friendship @user_id=1 @friend_id=2>]
>> @ted.friends
NoMethodError: undefined method `name' for nil:NilClass
	from /Library/Ruby/Gems/1.8/gems/dm-core-0.10.0/lib/dm-core/property_set.rb:150:in `initialize'
	from /Library/Ruby/Gems/1.8/gems/dm-core-0.10.0/lib/dm-core/property_set.rb:150:in `map'
	from /Library/Ruby/Gems/1.8/gems/dm-core-0.10.0/lib/dm-core/property_set.rb:150:in `initialize'
	from /Library/Ruby/Gems/1.8/gems/dm-core-0.10.0/lib/dm-core/associations/many_to_many.rb:23:in `new'
	from /Library/Ruby/Gems/1.8/gems/dm-core-0.10.0/lib/dm-core/associations/many_to_many.rb:23:in `target_key'
	from /Library/Ruby/Gems/1.8/gems/dm-core-0.10.0/lib/dm-core/associations/relationship.rb:161:in `query_for'
	from /Library/Ruby/Gems/1.8/gems/dm-core-0.10.0/lib/dm-core/associations/one_to_many.rb:42:in `collection_for'
	from /Library/Ruby/Gems/1.8/gems/dm-core-0.10.0/lib/dm-core/associations/one_to_many.rb:131:in `lazy_load'
	from /Library/Ruby/Gems/1.8/gems/dm-core-0.10.0/lib/dm-core/associations/one_to_many.rb:61:in `get'
	from /Library/Ruby/Gems/1.8/gems/dm-core-0.10.0/lib/dm-core/associations/one_to_many.rb:96:in `friends'
	from (irb):6
>>