has_one through doesn't work on new record

Here is example:

class Organization < ActiveRecord::Base
  has_many :members
end
class Member < ActiveRecord::Base
  has_one :organization # or belongs_to
end
class MemberHouse < ActiveRecord::Base
  has_one :member # or belongs_to
  has_one :organization, :through => :member
end

Then...

@organization = Organization.last
@member = @organiation.members.last

@member_house = MemberHouse.new(:address => "87 Worship Street")
@member_house.member = @member

In this situation, I expect that @member_house.organization returns @organization; but it returns nil actually.