DataMapper self-referential many-many

datamapper_user_following.rb

require 'dm-core'
DataMapper::Logger.new($stdout, :debug)
DataMapper.setup(:default, 'sqlite3::memory:')

class User
  include DataMapper::Resource
  
  has n, :follows
  has n, :followings, :model => 'Follow', :child_key => [:target_id]

  # can't get it to work
  has n, :targets, :through => :follows, :model => 'User'
end

class Follow
  include DataMapper::Resource
  
  belongs_to :user
  belongs_to :target, :model => 'User'
end

DataMapper.auto_migrate!

problem

>> user = User.create
>> user.targets
 ~ SELECT "users"."id", "users"."screen_name", "users"."full_name", "users"."created_at", "users"."updated_at", "users"."avatar_url", "users"."followers", "users"."following", "users"."tweets" FROM "users"
   INNER JOIN "follows" ON "users"."id" = "follows"."target_id"
   INNER JOIN "users" ON "follows"."user_id" = "users"."id"
   WHERE "follows"."user_id" = 1
   GROUP BY "users"."id", "users"."screen_name", "users"."full_name", "users"."created_at", "users"."updated_at", "users"."avatar_url", "users"."followers", "users"."following", "users"."tweets" ORDER BY "users"."id"

ERROR: ambiguous column name