title: Active Admin: add a form to new page
body: <p>I created a new page with Active Admin.
  It currently just shows a table which lists all entries of the model "HotelRoom" for the current date.</p>
  
  <p>Now I want to add a form where I can choose the Date for the data displayed in the table.
  But I can't get my head around how to add the form.</p>
  
  <p>Forms usually look like this:</p>
  
  <pre><code>&lt;% form_for @hotel_rooms do |f| %&gt;
    &lt;p&gt;
      &lt;%= f.label :date %&gt;&lt;br /&gt;
      &lt;%= f.text_field :date %&gt;
    &lt;/p&gt;
    &lt;p&gt;&lt;%= f.submit "Submit" %&gt;&lt;/p&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>and then I want to add a datepicker to the textfield like described here <a href="http://railscasts.com/episodes/213-calendars" rel="nofollow">http://railscasts.com/episodes/213-calendars</a>
  But I always get the error:</p>
  
  <pre><code>undefined method `model_name' for NilClass:Class
  </code></pre>
  
  <p>What am I doing wrong?</p>
  
  <p>EDIT: I tried adding the variable to the controller like this in the custom page file</p>
  
  <pre><code> controller do
    def index
      @hotel_rooms = HotelRoom.all
    end
   end
  </code></pre>
  
  <p>but still no luck. Same error.</p>