add method copy to purchase

purchase.py

@@ -28,6 +28,25 @@ class Purchase(metaclass=PoolMeta):
                 'depends': ['invoice_state'],}   
         })
 
+    @classmethod
+    def copy(cls, purchases, default=None):
+        if default is None:
+            default = {}
+        else:
+            default = default.copy()
+
+        default.setdefault('number', None)
+        default.setdefault('invoice_state', 'none')
+        default.setdefault('invoices_ignored', None)
+        default.setdefault('moves', None)
+        default.setdefault('shipment_state', 'none')
+        default.setdefault('purchase_date', None)
+        default.setdefault('quoted_by')
+        default.setdefault('confirmed_by')
+        default.setdefault('equipment_create', None)
+
+        return super(Purchase, cls).copy(purchases, default=default)
+
     @classmethod
     @ModelView.button
     def create_equipments(cls, purchases):