catch bulkwriteerror

mongoengine/queryset/base.py

@@ -371,6 +371,11 @@ class BaseQuerySet(object):
                 message = u'Tried to save duplicate unique keys (%s)'
                 raise NotUniqueError(message % six.text_type(err))
             raise OperationError(message % six.text_type(err))
+        except pymongo.error.BulkWriteError as err:
+            # inserting documents that already have an _id field will
+            # give huge performance debt or raise
+            message = u'Document must not have _id value before bulk write (%s)'
+            raise NotUniqueError(message % sx.text_type(err))
 
         if not load_bulk:
             signals.post_bulk_insert.send(