Write the right cost to the right product in recompute_cost_price

As some products can be skipped if they are services, the zip loop can be
shifted. So it is replaced by a dictionary which have also the advantage to
group the write of the same cost.
Also if all the products are services, the call to write is empty which is not
allowed.

issue5218
review22681002

product.py

@@ -2,6 +2,7 @@
 # this repository contains the full copyright notices and license terms.
 import datetime
 from decimal import Decimal
+from collections import defaultdict
 
 from sql import Literal, Null
 from sql.aggregate import Max
@@ -188,28 +189,30 @@ class Product(object, StockMixin):
         pool = Pool()
         Template = pool.get('product.template')
 
-        costs = []
-        for product in products:
-            if product.type == 'service':
-                continue
-            costs.append(getattr(product, 'recompute_cost_price_%s' %
-                product.cost_price_method)())
-
         if hasattr(cls, 'cost_price'):
             digits = cls.cost_price.digits
             write = cls.write
-            records = products
+            record = lambda p: p
         else:
             digits = Template.cost_price.digits
             write = Template.write
-            records = [p.template for p in products]
+            record = lambda p: p.template
 
-        costs = [c.quantize(
-            Decimal(str(10.0 ** -digits[1]))) for c in costs]
+        costs = defaultdict(list)
+        for product in products:
+            if product.type == 'service':
+                continue
+            cost = getattr(product,
+                'recompute_cost_price_%s' % product.cost_price_method)()
+            cost = cost.quantize(Decimal(str(10.0 ** -digits[1])))
+            costs[cost].append(record(product))
+
+        if not costs:
+            return
 
         to_write = []
-        for record, cost in zip(records, costs):
-            to_write.append([record])
+        for cost, records in costs.iteritems():
+            to_write.append(records)
             to_write.append({'cost_price': cost})
 
         # Enforce check access for account_stock*