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

Question

I've seen a lot of posts about how to map 2 form fields to one model field, but what about mapping 1 form field to provide answers for two (or more) model fields? I have a form for users which gives a field, last_name, for a user. But I want the default password for that user to also be last_name (and I have password_confirmation set up, so that also needs to be last_name). How would I do this?
Form:
<div class="row">
<div class="col-md-6 col-md-offset-3">
<%= form_for(#user) do |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> </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 to the 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

Answer 1

I would add a before_validation callback to the User model like 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

Answer 2

If this will be your default and you don't want the user to be able to override it on creation, you could set it on the controller:
View:
<%= f.text_field :last_name, class: 'form-control' %>
Controller:
def create
#user = User.new(user_params)
#user.password = #user.last_name
#user.password_confirmation = #user.last_name
if #user.save
log_in #user
current_user
flash[:success] = "Welcome to the Penn Model Congress!"
redirect_to after_sign_in_path
else
render 'new'
end
end