title: Create empty queryset by default in django form fields
body: <p>I have this fields in form:</p>
  
  <pre><code>city = forms.ModelChoiceField(label="city", queryset=MyCity.objects.all())
  district = forms.ModelChoiceField(label="district", queryset=MyDistrict.objects.all())
  area = forms.ModelChoiceField(label="area", queryset=MyArea.objects.all())
  </code></pre>
  
  <p>district comes from click on city and area comes from click on area. With  <code>queryset=MyDistrict.objects.all()</code> and <code>queryset=MyArea.objects.all()</code> form will be very heavy. How can I make querysets empty by default?</p>