class ActiveRecord::Associations::HasOneThroughAssociation

def create_through_record(record)
  ensure_not_nested
  through_proxy  = owner.association(through_reflection.name)
  through_record = through_proxy.send(:load_target)
  if through_record && !record
    through_record.destroy
  elsif record
    attributes = construct_join_attributes(record)
    if through_record
      through_record.update(attributes)
    elsif owner.new_record?
      through_proxy.build(attributes)
    else
      through_proxy.create(attributes)
    end
  end
end

def replace(record)
  create_through_record(record)
  self.target = record
end