Reset purchase_requests field on copy

issue8719
review266181002

purchase.py

@@ -558,6 +558,15 @@ class PurchaseRequisitionLine(sequence_ordered(), ModelSQL, ModelView):
             origin=self,
             )
 
+    @classmethod
+    def copy(cls, lines, default=None):
+        if default is None:
+            default = {}
+        else:
+            default = default.copy()
+        default.setdefault('purchase_requests')
+        return super(PurchaseRequisitionLine, cls).copy(lines, default=default)
+
 
 class PurchaseRequest(metaclass=PoolMeta):
     __name__ = 'purchase.request'