diff --git a/invoice.py b/invoice.py
index d04b3a367d..9ffc768b5f 100644
--- a/invoice.py
+++ b/invoice.py
@@ -1,5 +1,7 @@
 #This file is part of Tryton.  The COPYRIGHT file at the top level of
 #this repository contains the full copyright notices and license terms.
+from functools import wraps
+
 from trytond.model import ModelView, Workflow, fields
 from trytond.pool import Pool, PoolMeta
 from trytond.transaction import Transaction
@@ -9,6 +11,18 @@ __all__ = ['Invoice', 'InvoiceLine']
 __metaclass__ = PoolMeta
 
 
+def process_purchase(func):
+    @wraps(func)
+    def wrapper(cls, invoices):
+        pool = Pool()
+        Purchase = pool.get('purchase.purchase')
+        func(cls, invoices)
+        with Transaction().set_user(0, set_context=True):
+            Purchase.process([p for i in cls.browse(invoices)
+                    for p in i.purchases])
+    return wrapper
+
+
 class Invoice:
     __name__ = 'account.invoice'
     purchase_exception_state = fields.Function(fields.Selection([
@@ -84,22 +98,19 @@ class Invoice:
         return super(Invoice, cls).draft(invoices)
 
     @classmethod
-    def paid(cls, invoices):
-        pool = Pool()
-        Purchase = pool.get('purchase.purchase')
-        super(Invoice, cls).paid(invoices)
-        with Transaction().set_user(0, set_context=True):
-            Purchase.process([p for i in cls.browse(invoices)
-                    for p in i.purchases])
+    @process_purchase
+    def post(cls, invoices):
+        super(Invoice, cls).post(invoices)
 
     @classmethod
+    @process_purchase
+    def paid(cls, invoices):
+        super(Invoice, cls).paid(invoices)
+
+    @classmethod
+    @process_purchase
     def cancel(cls, invoices):
-        pool = Pool()
-        Purchase = pool.get('purchase.purchase')
         super(Invoice, cls).cancel(invoices)
-        with Transaction().set_user(0, set_context=True):
-            Purchase.process([p for i in cls.browse(invoices)
-                    for p in i.purchases])
 
 
 class InvoiceLine:
diff --git a/tests/scenario_purchase.rst b/tests/scenario_purchase.rst
index 72fc513687..8eed003955 100644
--- a/tests/scenario_purchase.rst
+++ b/tests/scenario_purchase.rst
@@ -282,6 +282,9 @@ Open supplier invoice::
     u'in_invoice'
     >>> len(invoice.lines)
     2
+    >>> for line in invoice.lines:
+    ...     line.quantity = 1
+    ...     line.save()
     >>> invoice.invoice_date = today
     >>> invoice.save()
     >>> Invoice.post([invoice.id], config.context)
@@ -290,13 +293,22 @@ Open supplier invoice::
     u'posted'
     >>> payable.reload()
     >>> (payable.debit, payable.credit) == \
-    ... (Decimal('0.00'), Decimal('25.00'))
+    ... (Decimal('0.00'), Decimal('10.00'))
     True
     >>> expense.reload()
     >>> (expense.debit, expense.credit) == \
-    ... (Decimal('25.00'), Decimal('0.00'))
+    ... (Decimal('10.00'), Decimal('0.00'))
     True
 
+Check second invoices::
+
+    >>> config.user = purchase_user.id
+    >>> purchase.reload()
+    >>> len(purchase.invoices)
+    2
+    >>> sum(l.quantity for i in purchase.invoices for l in i.lines)
+    5.0
+
 Create a Return::
 
     >>> config.user = purchase_user.id
@@ -357,10 +369,10 @@ Open supplier credit note::
     >>> credit_note.state
     u'posted'
     >>> payable.reload()
-    >>> (payable.debit, payable.credit) == (Decimal(20), Decimal(25))
+    >>> (payable.debit, payable.credit) == (Decimal(20), Decimal(10))
     True
     >>> expense.reload()
-    >>> (expense.debit, expense.credit) == (Decimal(25), Decimal(20))
+    >>> (expense.debit, expense.credit) == (Decimal(10), Decimal(20))
     True
 
 Mixing return and purchase::
@@ -443,10 +455,10 @@ Checking the invoice::
     >>> mix_credit_note.state
     u'posted'
     >>> payable.reload()
-    >>> (payable.debit, payable.credit) == (Decimal(30), Decimal(60))
+    >>> (payable.debit, payable.credit) == (Decimal(30), Decimal(45))
     True
     >>> expense.reload()
-    >>> (expense.debit, expense.credit) == (Decimal(60), Decimal(30))
+    >>> (expense.debit, expense.credit) == (Decimal(45), Decimal(30))
     True
 
 Mixing stuff with an invoice method 'on shipment'::