Using model's sequence next value before inserting for special manipulation.

client.rb

# id : serial
# check_digit : integer

class Client

  set_primary_key :id
  
  before_create :define_check_digit

  private

  def self.next_id
    Integer(connection.select_value "SELECT nextval('#{sequence_name}')")
  end

  def define_check_digit
    self.id = self.class.next_id
    self.check_digit = Digit.calc id
  end
end