Dumping PyMongo cursor does not serialize first item

[Mise]

We are currently trying to upgrade Marshmallow from version 1.1.0 to the latest version. As our internal tests started failing we discovered a commit that breaks serializing PyMongo cursors. The commit is 48903a3.

Changing obj_prototype = obj[0] to obj_prototype = next(iter(obj)) causes the cursor position to change. When the serialization in the subsequent steps progresses the first item will not be serialized (as the cursor now points to the next item).

Workarounds:

• Converting the cursor to a list before passing it to dump().
• Setting update_fields = False when calling dump().