Mongdb combined limit and sort when using find function - mongodb

Question

I have db mongdb example with document a and document b
a_id type
1 1
2 2
3 3
4 4
Now. I want to extract the last N (1,2,3,4,5,....) values in table b in the same order as in the example above. But if I use skip function :
b.find().skip(M)
if M > N then result empty => wrong. I want dynamic M.
If I use sort and limit then it does not give the correct order.
b.find().sort({$natural:-1}).limit(M)
result:
4 4
3 3
I want a solution!

Answer 1

You can use the same skip() to access the last N documents in the collection.
N = Last N documents to be accessed
So the query is
b.find().skip(b.count() - N).pretty()
or you can play with the mongo shell just as javascript like
var totalCount = b.count()
db.find().skip(totalCount - N).pretty()