> db.textarea.vermongo.find();
{ "_id" : { "_id" : "y", "_version" : 1 }, "name" : "Thilo Planz", "_version" : 1 }

> db.textarea.vermongo.find( {'_id': {'_id': 'y', '_version': 1 }});
{ "_id" : { "_id" : "y", "_version" : 1 }, "name" : "Thilo Planz", "_version" : 1 }

explain =>
	"cursor" : "BtreeCursor _id_",

> db.textarea.vermongo.find( {'_id._id': 'y'});

explain =>
       "cursor" : "BasicCursor",

When using a BSONObject as the _id for the record, querying against only a part of the object should still use the _id btree index. I appreciate that because the existence and order of fields in the nested document is not known, this will not be a range scan (not even for an _id field of the nested document), but a "fast-full-index-scan" type of access path should be possible and faster than a table scan.