Django QuerySet union抛出TypeError

Question

我正在尝试使用QuerySet填充Django下拉列表。

形式

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)

两个＆＃39; prop1_names＆＃39;和＆＃39; prop2_names＆＃39;查询集基于相同的模型，并且prop1_name和prop2_name列都具有相同的数据类型。错误讯息：

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. 

感谢任何帮助。