Hello,

I encountered a strange behavior in Mongo 2.6.x. i am using Mongoid in ruby talk to MongoDB and Mongoid allows having default sort command. When I run the following command :

Person.find("someindex") in Mongoid, it is translated into
db.persons.find(

).sort(created_at: -1)

Mongoid is not really smart by generating such query because we know we will get either 0 or 1 result, so the sort operator does not make sense.

However, I've found cases where MongoDB is even less smart and caches the wrong index, including the created_at field.

I would expect MongoDB to automatically use the _id index whenever it is present in the query, no matter other fields that are present in the query, in partcular sort commands.

I already had a discussion about this on google group:
https://groups.google.com/forum/#!searchin/mongodb-user/rpechayr/mongodb-user/-btb_43ONfk/w0dMlECPInIJ