Question

As far as I know, I can’t specify a complex sort function in Mongo so I have to use a map/reduce query which feels a bit useless since I don’t even use the reduce portion. This is fine but the collection it returns is too large, I need to sort and limit it (paginate). Is it possible to do that with an inline call? I can’t store the resulting collection and then query it because the collection is unique to each user and constantly changing.

Answer 1

Normally sort in mongodb mapreduce applied before the map, literally they are applied to the input data not the output.

From mongodb docs

   [, sort : <sorts the input objects using this key. Useful for optimization, like sorting by the emit key for fewer reduces>]
   [, limit : <number of objects to return from collection>]

So if you want to sort & limit the map reduce output, you must store the result in temp collection. I dont think its possible to apply the sort & limit to the inline mapreduce output since it runs on RAM.

EDIT:

There is a open issue (Add support for sorting and limiting inline map/reduce) in mongodb Jira regarding this. Have a look.