gistfile1.rbx

father = Father.new

child = Child.new(:father => father)

father.save

child.father_id.should == father.id  # how? the child references the father, but there's no backlink atm

# ... unless

class Child
  def father
    return @father if @father

    if @father_id
      self.father = Father.get(@father_id)
    end

    @father
  end

  def father=(father)
    @father = father
    @father_id = nil  # @father should be authoritative
  end

  def father_id
    @father.id || @father_id
  end

  def father_id=(father_id)
    return if @father && @father_id == @father.id  # do nothing if no change

    if @father && @father_id != @father.id
      @father = nil
    end

    @father_id = father_id
  end

  # ...
end

# now changing the father object will update the father_id key