Implementing geospatial queries in java

[pinaki ghosh]

I want to replicate the following query in Java

db.airports.find(
{
  loc : {
    $near : {
      $geometry : {
        type : "Point" ,
        coordinates : [-73.965355,40.782865]
      },
      $maxDistance : 20000
    }
  },
  type : "International"
},
{
  name : 1,
  code : 1,
  _id : 0
}
);

I tried

BasicDBList geoCoord = new BasicDBList();
geoCoord.add(lng);
geoCoord.add(lat);
BasicDBObject geometry = new BasicDBObject("type", "Point");
geometry.append("coordinates", geoCoord);
BasicDBObject near = new BasicDBObject("$geometry", geometry);
BasicDBObject query = new BasicDBObject();
query.append("$maxDistance", distance);
query.append("$nearSphere", near);

 but it throws me this error

com.mongodb.CommandFailureException: { "serverUsed" : "127.0.0.1:27017" , "errmsg" : "exception: unknown top level operator: $maxDistance" , "code" : 2 , "ok" : 0.0}

I have indexed the location column.

[Will Berkeley]

The error indicates you constructed the query inappropriately, with $maxDistance out of place. If I translate query into mongo shell BSON syntax, I believe it's

{
    "$maxDistance" : distance,    // :(
    "$nearSphere" : {
        "$geometry" : {
            "type" : "Point",
            "coordinates" : [lng, lat]
       }
    }
}

The problem is caused by $maxDistance being at the top level rather than inside $near/$nearSphere. I believe the correct code to construct the geo query is

BasicDBList geoCoord = new BasicDBList();
geoCoord.add(lng);
geoCoord.add(lat);
BasicDBObject geometry = new BasicDBObject("type", "Point");
geometry.append("coordinates", geoCoord);
BasicDBObject near = new BasicDBObject("$geometry", geometry);
BasicDBObject query = new BasicDBObject();

near.append("$maxDistance", distance);        // CHANGED
query.append("$nearSphere", near);

-Will