old_contents: from django import forms
  
  from symposion.schedule.models import Slot, Presentation
  
  
  class SlotEditForm(forms.Form):
      
      presentation = forms.ModelChoiceField(
          queryset=Presentation.objects.filter(slot__isnull=True),
          required=True,
      )
      slot_pk = forms.CharField(
          max_length=10,
          widget=forms.HiddenInput,
          required=False,
      )
      
      def clean_slot_pk(self):
          value = self.cleaned_data["slot_pk"]
          try:
              Slot.objects.get(pk=value)
          except Slot.DoesNotExist:
              raise forms.ValidationError("Invalid slot.")
          return value
  
new_contents: from django import forms
  
  from symposion.schedule.models import Slot, Presentation
  
  
  class SlotEditForm(forms.Form):
      
      presentation = forms.ModelChoiceField(
          queryset=Presentation.objects.filter(slot__isnull=True),
          required=True,
      )
  
subject: remove slot pk from form