class UserField(forms.CharField): 
    class widget(forms.widgets.TextInput): 
        def render(self, name, value, attrs=None): 
            if isinstance(value, int): 
                value = unicode(User.objects.get(pk=value)) 
            return super(UserField.widget, self).render(name, value, attrs) 
        
    def clean(self, value): 
        value = super(UserField, self).clean(value) 
        if not value: 
            return None 
        try: 
            return User.objects.get(username=value) 
        except User.DoesNotExist: 
            raise forms.ValidationError(u'invalid user name')