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

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

      <ul>
      <% activity.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>
    <div class="form-group">
      <div class="row">
        <div class="col-md-6">
          <%= f.label :start %>
          <%= f.datetime_field :start, class: 'form-control' %>
        </div>
        <div class="col-md-6">
          <%= f.label :end %>
          <%= f.datetime_field :end, class: 'form-control' %>
        </div>
      </div>
    </div>
    <div class="form-group">
      <%= f.label :description %>
      <%= f.text_area :description, class: 'form-control' %>
    </div>
    <div class="form-group">
      <%= f.label :location %>
      <%= f.text_area :location, class: 'form-control' %>
    </div>
    <div class="form-group">
      <%= f.label :deadline %>
      <%= f.datetime_field :deadline, class: 'form-control' %>
    </div>
    <%= f.submit %>
  </div>
<% end %>