foo.rb

# A user have methods for login and such
class User < Sequel::Model(:users)
end

# A administrator is a USER but have extra data in
# administrators table.
class Administrator < User
  one_to_one :user
  set_dataset(:administrators)
end

# When creating a administrator
# i want one row in users and one row in administrators
# administrators: administrator_id, user_id, email
# users: user_id, username
a = Administrator.new
a.email = "foo@foo.bar"
a.username = "foobar"
a.save