'A suggested convenience function for subclasses.
NOTE: This does not add anything to the cart. That is left up to the
subclasses to implement for themselves'

@classmethod
def add_to_order(cls, order, *args, **kwargs):

    currency = kwargs.get('currency', 'usd')
    if ((order.currency != currency) and order.orderitem_set.exists()):
        raise InvalidCartItem(_('Trying    to    add    a    different    currency    into    the    cart'))
