edit.html.erb

<%= form_for @user do |f| %>
  <%= f.label "Widgets:" %>
  <%= f.text_field "x" %>
  <%= f.submit "Add widgets to User" %>
<% end %>

User.rb

class User < ActiveRecord::Base
  has_many :widgets
end

UsersController.rb

...

def edit
  @user = User.find(1)
end

def update
  # this method takes the value from the form
  # splits it into an array (split by commas)
  # and adds 1 widget per comma-separated word
end

...

Widget.rb

class Widget < ActiveRecord::Base
  belongs_to :user
end