datamapper association empty set strange behavior

require 'dm-core'
require 'dm-migrations'

class Foo
  include DataMapper::Resource
  property :id, Serial
  has n, :bars
end

class Bar
  include DataMapper::Resource
  property :id, Serial
  belongs_to :foo
  belongs_to :baz
end

class Baz
  include DataMapper::Resource
  property :id, Serial
  has n, :bars
end

url = "sqlite3::memory:"

DataMapper.setup(:default, url)
DataMapper.finalize
DataMapper.auto_migrate!


require 'minitest/spec'
require 'minitest/autorun'
 
describe Foo do
  foo = Foo.create()
  baz = Baz.create()
  bar = Bar.create(:baz => baz , :foo => foo)
  
  describe "chained associations" do
    it "should return all results when chaining with all" do
      foo.bars.baz.count.must_equal 1
    end
    
    it "should return no results when chaining with an empty set" do
      foo.bars(:id => []).baz.count.must_equal 0
    end
  end

end