ruby - Rails map one field in form to two fields in model -

i've seen lot of posts how map 2 form fields 1 model field, mapping 1 form field provide answers 2 (or more) model fields? have form users gives field, last_name, user. want default password user last_name (and have password_confirmation set up, needs last_name). how this?

form:

<div class="row">   <div class="col-md-6 col-md-offset-3">     <%= form_for(@user) |f| %>        <%= f.label :first_name %>       <%= f.text_field :first_name, class: 'form-control' %>        <%= f.label :last_name %>       <%= f.text_field :last_name, :password, :password_confirmation,                          class: 'form-control' %>        <%= f.label :email %>       <%= f.email_field :email, class: 'form-control' %>        <%= f.hidden_field :access_level, :value => "chair" %>        <%= f.label :phone %>       <%= f.text_field :phone_number, :id => "phone", class: 'form-control' %>         <p>&nbsp;</p>       <div class="col-md-12">         <%= f.submit "add chair", class: "btn btn-primary" %>       </div>     <% end %>   </div> </div> 

user_controller.rb

  def create     @user = user.new(user_params)     if @user.save       log_in @user       current_user       flash[:success] = "welcome penn model congress!"       redirect_to after_sign_in_path     else       render 'new'     end   end    private      def user_params       params.require(:user).permit(:first_name, :last_name, :email, :password,                                    :password_confirmation, :access_level,                                     :phone_number)     end end 

i add before_validation callback user model this:

# in app/models/user.rb before_validation :set_default_password  private   def set_default_password     self.password              ||= last_name     self.password_confirmation ||= last_name   end