old_contents: from django import forms
  
  from .proxies import PlanProxy
  
  
  class PlanForm(forms.Form):
      plan = forms.ModelChoiceField(queryset=PlanProxy.objects.all)
  
new_contents: from django import forms
  
  from .proxies import PlanProxy
  
  
  class PlanForm(forms.Form):
      plan = forms.ModelChoiceField(queryset=PlanProxy.objects.all())
  
subject: Fix bug in PlanForm