django forms exclude values from choicefield

I want to exclude those options from the ChoiceField in the forms that has been already selected and are into the database. Below is the Code.

models.py

SEMESTER_CHOICES = (      ("1", "1"),      ("2", "2"),      ("3", "3"),      ("4", "4"),      ("5", "5"),      ("6", "6"),      ("7", "7"),      ("8", "8"),  )  class Publish(models.Model):      dates = models.CharField(max_length=255, choices = SEMESTER_CHOICES)        def __str__(self):          return self.dates  

form.py

  SEMESTER_CHOICES = (      ("1", "1"),      ("2", "2"),      ("3", "3"),      ("4", "4"),      ("5", "5"),      ("6", "6"),      ("7", "7"),      ("8", "8"),  )    class PublishForm(forms.ModelForm):          def __init__(self, **kwargs):          super(PublishForm, self).__init__(self, **kwargs)          selected_choices = [i.dates for i in Publish.objects.all()]                  self.fields['dates'].choices = ({(k, v) for k, v in SEMESTER_CHOICES                                               if k not in selected_choices})        class Meta:          model = Publish          fields = ['dates']  

view.py

def add(request):      if request.method == 'POST':          form = PublishForm(request.POST)          if form.is_valid():              form.save()              return redirect('mainapp:add')      else:          form = PublishForm()        context = {          'form' : form      }      return render(request, 'mainapp/add.html', context)  

What am I doing wrong? Ive followed these 2 links here. link1 and link2. Any suggestions please.

Also I am getting the error : 'PublishForm' object has no attribute 'get'

Thank you

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