how to get the last N records in MongoDB

.sort()

db.foo.find().sort({x:1});

The 1 will sort ascending (oldest to newest) and -1 will sort descending (newest to oldest.)

If you use the auto created _id field it has a date embedded in it … so you can use that to order by …

db.foo.find().sort({_id:1});

That will return back all your documents sorted from oldest to newest.

Natural Order

You can also use a Natural Order mentioned above …

db.foo.find().sort({$natural:1});

Again, using 1 or -1 depending on the order you want.

Use .limit()

Lastly, it’s good practice to add a limit when doing this sort of wide open query so you could do either …

db.foo.find().sort({_id:1}).limit(50);

db.foo.find().sort({$natural:1}).limit(50);

References
http://stackoverflow.com/questions/4421207/mongodb-how-to-get-the-last-n-records