## old db base
class OldServerConnectionBase < ActiveRecord::Base
  establish_connection(
    :adapter  => "mysql",
    :host     => "localhost",
    :username => "user",
    :password => "somepwd",
    :database => "old_db_name"
  )
  class MethodNotAvailable<StandardError; end;
  def delete(id)
    raise MethodNotAvailable
  end
  def delete_all(conditions =nil)
    raise MethodNotAvailable
  end
  def destroy(id)
    raise MethodNotAvailable
  end
  def destroy_all(conditions = nil)
    raise MethodNotAvailable
  end
  def create_or_update
    raise MethodNotAvailable
  end
  def update_attribute(name, value)
    raise MethodNotAvailable
  end
  def update_attributes(attributes)
    raise MethodNotAvailable
  end
end

## some old db model
class OldBook < OldServerConnectionBase
  set_table_name 'my_app_book_table'
  set_primary_key 'id_book'


  def self.migrate_all
    find(:all).each(&:migrate_me)
  end

  def migrate_me
    new_book = Book.new()
    new_book.isbn_10 = self.isbn
    new_book.isbn_13 = convert_to_isbn_13(self.isbn)
    new_book.save!
  end
end

## in your console you just call
OldBook.migrate_all