aardbei/app/views/groups/_form.html.erb

<%= form_for(group) do |f| %>
  <% if group.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(group.errors.count, "error") %> prohibited this group from being saved:</h2>

      <ul>
      <% group.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="actions">
    <div class="form-group">
      <%= f.label :name %>
      <%= f.text_field :name, class: 'form-control' %>
    </div>
    <%= f.submit class: 'btn btn-primary' %>
  </div>
<% end %>