diff --git a/invoice.py b/invoice.py
index 790d667b1c..13b17f2d1d 100644
--- a/invoice.py
+++ b/invoice.py
@@ -31,7 +31,8 @@ class Invoice(metaclass=PoolMeta):
         ('ignored', 'Ignored'),
         ('recreated', 'Recreated'),
         ], 'Exception State'), 'get_sale_exception_state')
-    sales = fields.Function(fields.One2Many('sale.sale', None, 'Sales'),
+    sales = fields.Function(fields.Many2Many(
+            'sale.sale', None, None, "Sales"),
         'get_sales', searcher='search_sales')
 
     def get_sale_exception_state(self, name):
diff --git a/sale.py b/sale.py
index a0a8b16733..19fed9721f 100644
--- a/sale.py
+++ b/sale.py
@@ -187,8 +187,9 @@ class Sale(Workflow, ModelSQL, ModelView, TaxableMixin):
             ('paid', 'Paid'),
             ('exception', 'Exception'),
             ], 'Invoice State', readonly=True, required=True)
-    invoices = fields.Function(fields.One2Many('account.invoice', None,
-            'Invoices'), 'get_invoices', searcher='search_invoices')
+    invoices = fields.Function(fields.Many2Many(
+            'account.invoice', None, None, "Invoices"),
+        'get_invoices', searcher='search_invoices')
     invoices_ignored = fields.Many2Many('sale.sale-ignored-account.invoice',
             'sale', 'invoice', 'Ignored Invoices', readonly=True)
     invoices_recreated = fields.Many2Many(
@@ -210,10 +211,11 @@ class Sale(Workflow, ModelSQL, ModelView, TaxableMixin):
             ('sent', 'Sent'),
             ('exception', 'Exception'),
             ], 'Shipment State', readonly=True, required=True)
-    shipments = fields.Function(fields.One2Many('stock.shipment.out', None,
-        'Shipments'), 'get_shipments', searcher='search_shipments')
-    shipment_returns = fields.Function(
-        fields.One2Many('stock.shipment.out.return', None, 'Shipment Returns'),
+    shipments = fields.Function(fields.Many2Many(
+            'stock.shipment.out', None, None, "Shipments"),
+        'get_shipments', searcher='search_shipments')
+    shipment_returns = fields.Function(fields.Many2Many(
+            'stock.shipment.out.return', None, None, "Shipment Returns"),
         'get_shipment_returns', searcher='search_shipment_returns')
     moves = fields.One2Many('stock.move', 'sale', 'Moves', readonly=True)
     origin = fields.Reference('Origin', selection='get_origin', select=True,