class ActiveRecord::Associations::HasOneThroughAssociation

def create_through_record(record, save)
  ensure_not_nested
  through_proxy  = through_association
  through_record = through_proxy.load_target
  if through_record && !record
    through_record.destroy
  elsif record
    attributes = construct_join_attributes(record)
    if through_record && through_record.destroyed?
      through_record = through_proxy.tap(&:reload).target
    end
    if through_record
      if through_record.new_record?
        through_record.assign_attributes(attributes)
      else
        through_record.update(attributes)
      end
    elsif owner.new_record? || !save
      through_proxy.build(attributes)
    else
      through_proxy.create(attributes)
    end
  end
end

def replace(record, save = true)
  create_through_record(record, save)
  self.target = record
end