validation - Rails does not add error classes to inputs -

i have rather basic form bunch of fields. here's excerpt:

<%= form_for(@subject) |f| %>    <% if @subject.errors.any? %>     <div id="error_explanation">     <h2><%= t('activerecord.errors.subject.header', :model => subject.model_name.human, :count => @subject.errors.count) %></h2>        <ul>       <% @subject.errors.full_messages.each |msg| %>         <li><%= msg %></li>       <% end %>       </ul>     </div>   <% end %>    <div class="group">     <fieldset class="left">       <div class="field">         <%= f.label "anrede" %>          <%= f.radio_button :title, "herr" %>         <%= f.label :title_herr, "herr", class: "radio" %>          <%= f.radio_button :title, "frau" %>         <%= f.label :title_frau, "frau", class: "radio" %>       </div>       <div class="field">         <%= f.label :name %>         <%= f.text_field :firstname, class: "m", placeholder: "vorname" %>          <%= f.text_field :lastname, class: "m", placeholder: "nachname" %>       </div>       <div class="field">         <%= f.label "stadt" %>         <%= f.select :city, subject::cities, prompt: "- bitte wählen -" %>       </div> 

the controller still scaffold:

  # post /subjects   # post /subjects.json   def create     @subject = subject.new(subject_params)      respond_to |format|       if @subject.save         format.html { }         format.json { render action: 'create', status: :created, location: @subject }       else         format.html { render action: 'new' }         format.json { render json: @subject.errors, status: :unprocessable_entity }       end     end   end 

however when submitting invalid form, show #error_explanation-div, not wrap erroneous fields div.field_with_error once did. not sure change made break.

i not sure if behaviour expect available rails, surely can check out simple_form or formtastic gem (i'd recommend former)