request

{"utf8"=>"✓",
 "_method"=>"put",
 "authenticity_token"=>"...",
 "user"=>{"email"=>"email@email.com",
 "password"=>"[FILTERED]",
 "first_name"=>"Bob",
 "last_name"=>"Jones",
 "subscription"=>{"plan_id"=>"5"}},
 "password_confirmation"=>"[FILTERED]",
 "commit"=>"Update User",
 "id"=>"2"}

view

<%= f.fields_for @user.subscription_attributes do |fs| %>
          <div class='control-group<%= " error" if @user.subscription.errors.has_key?(:plan_id) %>'>
            <label class='control-label'>Plan</label>
            <div class='controls'>
              <%= fs.select :plan_id, @plans.map {|p| [p.plan_name, p.id] } %>
              <% if @user.subscription.errors.has_key?(:plan_id) %>
                  <span class='help-inline'>Plan <%= @user.subscription.errors.get(:plan_id).first %></span>
              <% end %>
            </div>
          </div>
      <% end %>