title: Django QuerySet union throws TypeError
body: <p>I am trying to populate a Django dropdown list using a QuerySet.</p>
  
  <p><strong>forms</strong></p>
  
  <pre><code>class PropertyForm(forms.Form):
      prop1_names = Property.objects.values_list('prop1_name', flat = True).order_by('-prop_score')[:1]
      prop2_names = Property.objects.values_list('prop2_name', flat = True).order_by('-prop_score')[:1]
      props_queryset = prop1_names | prop2_names
  
      properties = forms.ModelChoiceField(queryset = props_queryset)
  </code></pre>
  
  <p>Both 'prop1_names' and 'prop2_names' query sets are based on the same model and both columns prop1_name and prop2_name have the same data type. Error message:</p>
  
  <pre><code>File "/usr/local/lib64/python3.4/site-packages/django/db/models/query.py", line 1115, in _merge_sanity_check
      % self.__class__.__name__
  TypeError: Merging 'QuerySet' classes must involve the same values in each case. 
  </code></pre>
  
  <p>Appreciate any help.</p>