forms.py

# Is also used to change details of an order, if needed.
class NewOrChangeOrderForm(forms.Form):
    customer = forms.ModelChoiceField(queryset=Customer.objects.order_by('id'), empty_label='<choose>')
    # ...
    
    def __init__(self, *args, **kwargs):
        super(NewOrChangeOrderForm, self).__init__(*args, **kwargs)

        # Customise how we show customer name
        self.fields['customer'].label_from_instance = lambda obj: "%s (%s %s)" % (obj.name, obj.firstname, obj.lastname) if obj.nick else "%s %s" % (obj.firstname, obj.lastname)