[FIX] point_of_sale: match payment to order amount when invoicing

The POS generates journal entries for both the amount of money the
customer pays and the amount of money the POS user returns to the
customer (the 'change'). Whether or not this is the correct thing to do
is up for debate, but we can't change that in stable anyway.

One of the main issues is that in 9.0 accounting will propose
unreconciled payments belonging to the customer on open, unpaid
invoices. This does not work well with the account entries generated by
the POS. Accounting will propose the amount the customer paid, without
taking into account the change.

Example: a customer buys $7 worth of products and pays with a $10
bill. The customer gets $3 change. The invoice will propose the $10
payment.

When you add that payment to the invoice a residual amount ($3 in the
example) will remain on the paymentline, complicating everything even
more.

Although we cannot change how the POS does accounting in a stable
release, we at least want people to be able to easily handle invoices
generated through the POS. To achieve that this patch will make the
backend match the payment to the total amount of the invoice removing
any potential change.

opw-671486

addons/point_of_sale/point_of_sale.py

@@ -701,6 +701,23 @@ class pos_order(osv.osv):
 
             return new_session_id
 
+    def _match_payment_to_invoice(self, cr, uid, order, context=None):
+        account_precision = self.pool.get('decimal.precision').precision_get(cr, uid, 'Account')
+
+        # ignore orders with an amount_paid of 0 because those are returns through the POS
+        if not float_is_zero(order['amount_return'], account_precision) and not float_is_zero(order['amount_paid'], account_precision):
+            cur_amount_paid = 0
+            payments_to_keep = []
+            for payment in order.get('statement_ids'):
+                if cur_amount_paid + payment[2]['amount'] > order['amount_total']:
+                    payment[2]['amount'] = order['amount_total'] - cur_amount_paid
+                    payments_to_keep.append(payment)
+                    break
+                cur_amount_paid += payment[2]['amount']
+                payments_to_keep.append(payment)
+            order['statement_ids'] = payments_to_keep
+            order['amount_return'] = 0
+
     def _process_order(self, cr, uid, order, context=None):
         session = self.pool.get('pos.session').browse(cr, uid, order['pos_session_id'], context=context)
 
@@ -757,6 +774,10 @@ class pos_order(osv.osv):
         for tmp_order in orders_to_save:
             to_invoice = tmp_order['to_invoice']
             order = tmp_order['data']
+
+            if to_invoice:
+                self._match_payment_to_invoice(cr, uid, order, context=context)
+
             order_id = self._process_order(cr, uid, order, context=context)
             order_ids.append(order_id)