django form dropdown with all users in it

I want to create a form which has a dropdown with all users in it. I tried it like this, with no luck.

开发者_JS百科class ContactFilterByClassificationForm(forms.Form):
   kam = forms.ChoiceField(choices=User.objects.all())

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You want to use a ModelChoiceField.

class ContactFilterByClassificationForm(forms.Form):
    kam = forms.ModelChoiceField(queryset=User.objects.all())