aggregator.rb

class Profile < ActiveModel::Aggregator
  aggregate :user
  aggregate :email

  def combine
    user.save
    user.emails.build(email_attributes).save
  end
end

class ProfileController < ApplicationController
  def create
    @profile = Profile.new(profile_attributes)
    
    if @profile.save
      redirect_to somewhere
    else
      render :new
    end
  end
end

Aggregator could automatically expose its aggregates as prefixed attributes:

<%= form_for @profile do |f| %>
  <%= f.text_field :user_name %>
  <%= f.email_field :email_address %>
  <%= f.password_field :user_password %>
  <%= f.password_field :user_password_confimation %>
<% end%>

why have a separate api for combine, rather than just Profile having save? it is because you don't want to override a save method it would have inherited?