How to post prepopulated disabled ModelMultipleChoiceField?

Question

I need my ModelMultipleChoiceField to be prepopulated with query parameters from GET-request (/?extending=1&extending2). I know from this question that I can define get_initial() method for my view, returning my request. And everything works fine until I disable the ModelMultipleChoiceField, since I don't need it to be mutable.

This is my ModelForm class deduced from this answer:

class ExtensionForm(forms.ModelForm):
    class Meta:
        model = Record
        fields = ['extending', 'name', 'description', 'picture', 'content', 'files']

    extending = forms.ModelMultipleChoiceField(queryset=Record.objects.all())

    def __init__(self, *args, **kwargs):
        if kwargs.get('instance'):
            initial = kwargs.setdefault('initial', {})
            initial['extending'] = [el.pk for el in kwargs['instance'].extending.all()]

        forms.ModelForm.__init__(self, *args, **kwargs)
        self.fields['extending'].disabled = True

    def save(self, commit=True):
        instance = forms.ModelForm.save(self, False)

        old_save_m2m = self.save_m2m
        def save_m2m():
           old_save_m2m()
           instance.extending.clear()
           instance.extending.add(*self.cleaned_data['extending'])
        self.save_m2m = save_m2m

        if commit:
            instance.save()
            self.save_m2m()

        return instance

How do I update this class in order for my request to get there?