only need _id for exists

Author: Mike Dirolf

gridfs/__init__.py

@@ -215,8 +215,8 @@ def exists(self, document_or_id=None, **kwargs):
  .. versionadded:: 1.8
  """
  if kwargs:
- return self.__files.find_one(kwargs) is not None
- return self.__files.find_one(document_or_id) is not None
+ return self.__files.find_one(kwargs, ["_id"]) is not None
+ return self.__files.find_one(document_or_id, ["_id"]) is not None
 
  def open(self, *args, **kwargs):
  """No longer supported.