I have a compound key. Searching it directly works just fine.

find({_id : { a:'1', b:'2' }}) -> 1 doc

As I want to get a couple of documents in on query, varying on the second part of the key I tried

find({_id : { a:'1', b: { $in: ['2'] }}) -> 0 docs

while this was expected to give the very same result. After lots of tries I found that only

find({ _id: { $in: [

{ a: '1', b: '2' }

] ]}) -> 1 docs

works. Which essentially means one needs to send a lot of duplicated data in a query.

find({ _id: { $in: [

{ a: '1', b: '1' }

,

{ a: '1', b: '2' }

,

{ a: '1', b: '3' }

,

{ a: '1', b: '4' }

] ]}) -> 4 docs

Even worse - there is no error message or reason given why the $in operator does not work in compound keys.

Could be related to http://jira.mongodb.org/browse/SERVER-1026