Typecasting / SEL issue

gistfile1.rb

require 'rubygems'

gem 'dm-core', '0.10.0'
require 'dm-core'

class Post
  include DataMapper::Resource
  
  def self.default_repository_name
    :post
  end
  
  property :id, Serial
  property :title, String

  has n, :comments, :repository => :comment
end

class Comment
  include DataMapper::Resource
  
  def self.default_repository_name
    :comment
  end
  
  property :id, Serial
  property :body, Text
  property :post_id, String # for argument sake
  
  belongs_to :post, :repository => :post
end
  
DataMapper.setup(:comment, "sqlite3::memory:")
DataMapper.setup(:post,    :adapter => :in_memory)
DataObjects::Sqlite3.logger = DataObjects::Logger.new(STDOUT, 0)
DataMapper.auto_migrate!

Post.create(:title => 'Post 1')
Post.create(:title => 'Post 2')
 # !> method redefined; discarding old to_datetime
Comment.create(:body => 'first!', :post_id => '1')
Comment.create(:body => 'first!', :post_id => '2')

Comment.all.map { |comment| comment.post } # => [nil, nil]
# >> Thu, 25 Jun 2009 17:53:16 GMT ~ debug ~ (0.000511) SELECT sqlite_version(*)
# >> Thu, 25 Jun 2009 17:53:16 GMT ~ debug ~ (0.000399) DROP TABLE IF EXISTS "comments"
# >> Thu, 25 Jun 2009 17:53:16 GMT ~ debug ~ (0.000042) PRAGMA table_info("comments")
# >> Thu, 25 Jun 2009 17:53:16 GMT ~ debug ~ (0.001048) CREATE TABLE "comments" ("id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, "body" TEXT, "post_id" VARCHAR(50))
# >> Thu, 25 Jun 2009 17:53:16 GMT ~ debug ~ (0.000115) INSERT INTO "comments" ("body", "post_id") VALUES ('first!', '1')
# >> Thu, 25 Jun 2009 17:53:16 GMT ~ debug ~ (0.000057) INSERT INTO "comments" ("body", "post_id") VALUES ('first!', '2')
# >> Thu, 25 Jun 2009 17:53:16 GMT ~ debug ~ (0.000081) SELECT "id", "post_id" FROM "comments" ORDER BY "id"