MongoDB geospatial query with find() - mongodb

Question

I have some documents with a "loc" field that looks like this:
mongos> db.foo.findOne()
{
// ... some other stuff
"loc" : {
"type" : "Point",
"coordinates" : [
-83.362342,
26.687779
]
}
}
It's indexed like so:
mongos> db.foo.getIndices()
[
// ... some other stuff
{
"v" : 1,
"key" : {
"loc" : "2dsphere"
},
"name" : "loc_2dsphere",
"ns" : "amitest.foo",
"2dsphereIndexVersion" : 2
},
]
According to the docs I should be able to query it like the following, but I get the error shown:
mongos> db.foo.find(
{loc :
{$near:
{$geometry:
{type: "Point",
coordinates: [-83.362342, 26.687779]
},
$maxDistance: 100
}
}
}
).limit(5)
error: { "$err" : "use geoNear command rather than $near query", "code" : 13501 }
I can successfully query the field using runCommand(), but that's not ideal because I can't combine it with other criteria:
mongos> db.runCommand(
{geoNear : "foo",
near : {type : "Point",
coordinates : [-83.362342, 26.687779]
},
spherical : true,
maxDistance : 10,
limit : 5
}
)
I've got MongoDB 2.6.0, on a hash-sharded collection with 8 shards.

Answer 1

In the geospatial index documentation, it states that: "For sharded collections, queries using $near are not supported. You can instead use either the geoNear command or the $geoNear aggregation stage."
There is a bug related to this, https://jira.mongodb.org/browse/SERVER-926, which is marked as closed and targets a future release of MongoDB, 1.7.2, which will enable the $near operator to be used with sharded collections. However, note, there is a big but which is that near queries will be routed to all shards via mongos, which is likely to be very inefficient. This is because sharding is not supported on a geo column in general, see the related and still open bug https://jira.mongodb.org/browse/SERVER-1982
This, in turn, is related to to the fact that MongoDB uses geohashing to convert two dimensional geographical objects to something that can be both indexed using a B-tree and something that could, in theory, be used as a shard key. It is a difficult problem to fix, as with geohash indexes, objects very close together, can end up with hash values very far apart, and so it is difficult to design something that is both appropriate to use as a shard key, but also supports efficient geospatial querying such as $near. See the limitations section in the Wikipedia geohash article for more on potential issues with geohashing.