In one collection I have an embedded array of ObjectIds. The name of the array is "user_ids". I wrote a query to fetch all elements from the collection where the array has at least 1 element. The native MongoDB query looks like this:

db.mycollection.find( {"user_ids.0" :

} )

and it works fine. It returns 2 elements. In Java I did it like this:

query.put("user_ids.0", new BasicDBObject("$exists", Boolean.TRUE) );

Unfortunately it returns only 1 element. 1 is missing.
It looks like a bug.