MongoDB Geospatial Indexing

IOS开发中地图功能的后台处理：

radius的单位是度（°）一度是大约 111.12 公里
如果你需要查询一公里的之内的东西，如下：
center = [50, 50]
radius = 1/111.12
db.places.find({"loc" : {"$within" : {"$center" : [center, radius]}}}).

Play中:

public static Result getMomentAround(double radius, double latitude,
            double longitude) {

        // if u login
        if (!Security.isLogin())
            return ok(Pencho_tools.toJsend("fail", "4"));

        momentCollectionJackson.ensureIndex(new BasicDBObject("loc", "2d"),
                "geospacialIdx");
        LinkedList<Object> circle = new LinkedList<Object>();
        circle.addLast(new double[] { latitude, longitude });
        circle.addLast(radius/111.12);
        ArrayNode temp = new ArrayNode(null);
        BasicDBObject query = new BasicDBObject("loc", new BasicDBObject(
                "$within", new BasicDBObject("$center", circle)));
        DBCursor<D_moment> venue = momentCollectionJackson.find(query);
        while (venue.hasNext()) {
            temp.add(venue.next().toJson());
        }
        return ok(Pencho_tools.toJsend("success", "moment-list", temp));
    }