has_one. Deleting orphan tuples

Hi.

How to delete orphan tuples after a new object has been created through
has_one association?

Here's an example:

# a user model
class User < ActiveRecord::Base
has_one :account
end

# an account model
class Account < ActiveRecord::Base
belongs_to :user
end

# a users table
id = 1
name = Pavel

id = 2
name = Elena

# an accounts table
id = 1
user_id = 1
type = 'silver'

id = 2
user_id = 2
type = 'gold'

Then, if I create a new account for the first user an orphan tuple will
stay in the accounts table:

User.find(1).create_account(:type => 'gold')

# an accounts table
# an orphan tuple
id = 1
user_id = null
type = 'silver'

id = 2
user_id = 2
type = 'gold'

# a new tuple
id = 3
user_id = 1
type = 'gold'

So, how can I delete the tuple with id = 1 right after creating a new
account for the first user?

Thanks.

Ruby 1.9.2;
Rails 2.3.8.