Currently, users who wish to execute near queries against sharded collections must use either the geoNear command or the $geoNear aggregation stage. The $near and $nearSphere query predicates, on the other hand, will fail when issued against a sharded collection. We should add support for sharded $near/$nearSphere, especially given future plans to mark the query the geoNear command as deprecated.

One way to accomplish this would be to make use of the existing "geoNearDistance" $meta projection. In this design, the mongos query path would extend a $near query to project the "geoNearDistance" metadata as teh sort key. The query issued from mongos to the shards would look something like this:

> db.c.find({a: {$nearSphere: [1, 1]}}, {$sortKey: {$meta: "geoNearDistance"}})
{ "_id" : ObjectId("599c54f2c4ed9620c11bd27b"), "a" : [ 1, 1 ], "$sortKey" : 0 }
{ "_id" : ObjectId("599c577621d4e4bc06f8c498"), "a" : [ 1, 2 ], "$sortKey" : 0.0174532925199433 }

The AsyncResultsMerger, which merges query results on mongos, already knows how to interpret $sortKey in order to do a merge-sort.