MongoDB geospatial index, how to use it with array elements? - node.js

Question

I would like to get Kevin pub spots near a given position. Here is the userSpots collection :
{ user:'Kevin',
spots:[
{ name:'a',
type:'pub',
location:[x,y]
},
{ name:'b',
type:'gym',
location:[v,w]
}
]
},
{ user:'Layla',
spots:[
...
]
}
Here is what I tried :
db.userSpots.findOne(
{ user: 'Kevin',
spots: {
$elemMatch: {
location:{ $nearSphere: [lng,lat], $maxDistance: d},
type: 'pub'
}
}
},
},
function(err){...}
)
I get a strange error. Mongo tells me there is no index :2d in the location field. But when I check with db.userSpots.getIndexes(), the 2d index is there. Why doesn't mongodb see the index ? Is there something I am doing wrong ?
MongoError: can't find special index: 2d for : { spots: { $elemMatch: { type:'pub',location:{ $nearSphere: [lng,lat], $maxDistance: d}}}, user:'Kevin'}
db.userSpots.getIndexes() output :
{
"0" : {
"v" : 1,
"key" : {
"_id" : 1
},
"ns" : "mydb.userSpots",
"name" : "_id_"
},
"1" : {
"v" : 1,
"key" : {
"spots.location" : "2d"
},
"ns" : "mydb.usersBoxes",
"name" : "spots.location_2d",
"background" : true,
"safe" : null
}
}

Answer 1

For a similar geospatial app, I transformed the location into GeoJSON:
{
"_id" : ObjectId("5252cbdd9520b8b18ee4b1c3"),
"name" : "Seattle - Downtown",
"location" : {
"type" : "Point",
"coordinates" : [
-122.33145,
47.60789
]
}
}
(the coordinates are in longitude / latitude format. Mongo's use of GeoJSON is described here.).
The index is created using:
db.userSpots.ensureIndex({"location": "2dsphere"})
In my aggregation pipeline, I find matches using:
{"$match":{"location":{"$geoWithin": {"$centerSphere":[[location.coordinates[0], location.coordinates[1]], radius/3959]}}}}
(where radius is measured in miles - the magic number is used to convert to radians).

Answer 2

To index documents containing array of geo data MongoDB uses multi-key index. Multi-key index unwinds document to some documents with single value instead of array before indexing. So the index consider that key field as single value field not array.
Try query it without $elemMatch operator.