dm-redis-adapter with "through" associations

gistfile1.rb

require 'dm-core'
require 'dm-redis-adapter'

DataMapper.setup(:default, {:adapter  => "redis"})

class Book
  include DataMapper::Resource
  
  property :id,   Serial
  property :name, String
  
  has n, :book_tags
  has n, :tags, :through => :book_tags
end

class Tag
  include DataMapper::Resource
  
  property :id,   Serial
  property :name, String
  
  has n, :book_tags
  has n, :books, :through => :book_tags
end

class BookTag
  include DataMapper::Resource
  
  property :id,   Serial
  # property :book_id, Integer, :index => true
  # property :tag_id,  Integer, :index => true
  
  belongs_to :book, :key => true
  belongs_to :tag,  :key => true
end


Redis.new.flushdb

b = Book.create(:name => "Harry Potter")
t = Tag.create(:name => "fiction")

b.tags << t
b.save

b2 = Book.get(1)

p b2  # #<Book @id=1 @name="Harry Potter">
p b   # #<Book @id=1 @name="Harry Potter">
puts(b2 == b)  # true
p b.tags       # [#<Tag @id=1 @name="fiction">]
p b2.tags      # [] # !!!
p b2.book_tags # [] # !!!

# after uncommenting BookTag property declarations
# b2.book_tags is correct, but b2.tags still empty