Query an ArrayPosition in MongoDB - mongodb

Question

I have a collection like this:
> db.nodes.find()
{ "_id" : ObjectId("534d44e182bee8420ace927f"), "id" : "59598841", "created_by" : "JOSM", "geo" : { "type" : "Point", "coordinates" : [ 9.7346094, 52.371738 ] } }
{ "_id" : ObjectId("534d44e182bee8420ace9280"), "id" : "59598842", "created_by" : "JOSM", "geo" : { "type" : "Point", "coordinates" : [ 9.7343616, 52.3718121 ] } }
{ "_id" : ObjectId("534d44e182bee8420ace9281"), "id" : "59598845", "created_by" : "JOSM", "geo" : { "type" : "Point", "coordinates" : [ 9.7331504, 52.372057 ] } }
{ "_id" : ObjectId("534d44e182bee8420ace9282"), "id" : "59835778", "created_by" : "JOSM", "geo" : { "type" : "Point", "coordinates" : [ 9.7354137, 52.3711697 ] } }
{ "_id" : ObjectId("534d44e182bee8420ace9283"), "id" : "60409270", "created_by" : "JOSM", "geo" : { "type" : "Point", "coordinates" : [ 9.7354388, 52.3735999 ] } }
Now I want to query the coordinates-array to find the document with the greatest lon-value.
How can I do that, I have no idea :(
Tschüss, Andre

Answer 1

So actually getting the "lon" which is the first value, of the array may not seem immediately apparent, but is quite simple with aggregate:
db.nodes.aggregate([
{ "$project": {
"_id": {
"_id": "$_id",
"id": "$id",
"created_by": "$created_by",
"geo": "$geo",
},
"coordinates": "$geo.coordinates"
}},
{ "$unwind": "$coordinates" },
{ "$group": {
"_id": "$_id",
"lon": { "$first": "$coordinates" }
}},
{ "$sort": { "lon": 1 } },
{ "$limit": 1 },
{ "$project": {
"_id": "$_id._id",
"id": "$_id.id",
"created_by": "$_id.created_by",
"geo": "$_id.geo",
}}
])
Which gives the whole document with the higest value. Or if you just want the value:
db.nodes.aggregate([
{ "$unwind": "$geo.coordinates" },
{ "$group": {
"_id": "$_id",
"lon": { "$first": "$geo.coordinates" }
}},
{ "$group": {
"_id": null,
"lon": { "$max": "$lon" }
}}
])

Answer 2

Try using the aggregation framework
db.nodes.aggregate(
{ $unwind: "geo.coordinate" },
{ $group: { _id: { id: "$id"}, lon: { $first: "geo.coordinate" } } },
{ $group: { _id: null, maxval: { $max: "$lon" } } }
)
For more info on aggregation look here: http://docs.mongodb.org/manual/reference/operator/aggregation/