@http.route(['/shop/payment/transaction/<int:acquirer_id>'], type='json', auth="public", website=True)
    def payment_transaction(self, acquirer_id):
        """ Json method that creates a payment.transaction, used to create a
        transaction when the user clicks on 'pay now' button. After having
        created the transaction, the event continues and the user is redirected
        to the acquirer website.

        :param int acquirer_id: id of a payment.acquirer record. If not set the
                                user is redirected to the checkout page
        """
        cr, uid, context = request.cr, request.uid, request.context
        transaction_obj = request.registry.get('payment.transaction')
        order = request.website.sale_get_order(context=context)

        if not order or not order.order_line or acquirer_id is None:
            return request.redirect("/shop/checkout")

        assert order.partner_id.id != request.website.partner_id.id

        # find an already existing transaction
        tx = request.website.sale_get_transaction()
        if tx:
            tx_id = tx.id
            if tx.reference != order.name:
                tx = False
                tx_id = False
            elif tx.state == 'draft':  # button cliked but no more info -> rewrite on tx or create a new one ?
                tx.write({
                    'acquirer_id': acquirer_id,
                    'amount': order.amount_total,
                })
        if not tx:
            tx_id = transaction_obj.create(cr, SUPERUSER_ID, {
                'acquirer_id': acquirer_id,
                'type': 'form',
                'amount': order.amount_total,
                'currency_id': order.pricelist_id.currency_id.id,
                'partner_id': order.partner_id.id,
                'partner_country_id': order.partner_id.country_id.id,
                'reference': request.env['payment.transaction'].get_next_reference(order.name),
                'sale_order_id': order.id,
            }, context=context)
            request.session['sale_transaction_id'] = tx_id
            tx = transaction_obj.browse(cr, SUPERUSER_ID, tx_id, context=context)

        # update quotation
        request.registry['sale.order'].write(
            cr, SUPERUSER_ID, [order.id], {
                'payment_acquirer_id': acquirer_id,
                'payment_tx_id': request.session['sale_transaction_id']
            }, context=context)

        # confirm the quotation
        if tx.acquirer_id.auto_confirm == 'at_pay_now':
            request.registry['sale.order'].action_confirm(cr, SUPERUSER_ID, [order.id], context=request.context)

        return tx_id