title: cakePHP: form with related models not automagic updating
body: <p>I struggle with getting a form to work in the way I want it to behave.
  I have a Regions and Properties model, one region can have many properties and so on... </p>
  
  <p>I created a form to select the Region and then a Property in that region!</p>
  
  <p>The form is having both lists but I struggle to have the second list [Property] updating automagic with only the properties in the region you have selected from the first list [Regions]</p>
  
  <p>When you select a different region in the list, it should update automagic the property list, so you only see the properties for that region! Sorry for my bad explaining, but not sure how to explain this any better.</p>
  
  <p>This is the code in my controller:</p>
  
  <pre><code>// Retrieve the region list
          $this-&gt;set('regions', $this-&gt;Region-&gt;find('list', array(
              'fields' =&gt; array('Region.id', 'Region.regionname'),
              'order' =&gt; 'regionname',
              )));
          // Retrieve Property list for the regions.
          $this-&gt;set('properties', $this-&gt;Region-&gt;Property-&gt;find('list', array(
              'conditions' =&gt; array('Property.live' =&gt; true ),
              'fields' =&gt; array('Property.id','Property.description'),
              'order' =&gt; 'id',
              )));
  </code></pre>
  
  <p>This is part of my form.</p>
  
  <pre><code>&lt;?php echo $this-&gt;Form-&gt;create('Upload', array('action' =&gt; 'add', 'type' =&gt; 'file')); ?&gt;
  &lt;?php echo $this-&gt;Form-&gt;input('region_id', array('label' =&gt; 'Select Region:')); ?&gt;
  &lt;?php echo $this-&gt;Form-&gt;input('property_id', array('label' =&gt; 'Select Property:')); ?&gt;
  &lt;?php echo $this-&gt;Form-&gt;file('file'); ?&gt;
  </code></pre>
  
  <p>I have spend a lot of time looking around here and on youtube, but can't find it :-(</p>