model_unique_field.rb

# Add a unique index in your migrations like this:
# add_index :table, :code, unique: true

before_create :set_code
around_save :save_with_unique_code

def self.generate_code
  SecureRandom.hex(2).upcase
end
  
def set_code(force = false)
  self.code = self.class.generate_code if code.nil? || force
end

def save_with_unique_code
  begin
    self.class.connection.execute("SAVEPOINT before_record_create;")
    yield
  # NOTE: be careful if you have other unique fields!
  rescue ActiveRecord::RecordNotUnique
    set_code(true)
    self.class.connection.execute("ROLLBACK TO SAVEPOINT before_record_create;")
    retry
  end
end