mongoDB - average on array values

Question

I'm trying to compute an average aggregation operation on each values of an array for each documents in my collection.

Document structure

{
   myVar: myValue,
   [...]
   myCoordinates: [
      myLng,
      myLat
   ]
}

So, I tried to compute average of myLng and myLat values of myCoordinates array for the whole collection of documents by querying the collection like this :

myColl.aggregate([{
   $group: {
       _id: 0,
       lngAvg: { $avg: "$myCoordinates.0" },
       latAvg: { $avg: "$myCoordinates.1" }
   }
}])

But unfortunately, it doesn't work and returns me a value of 0 for both lngAvg and latAvg fields.

Have you some ideas? Is this feasible at least?

I think you need to $unwind the document first

Sammaye
– Sammaye

2013-06-25 16:22:02 +00:00
Commented Jun 25, 2013 at 16:22 — Sammaye
– Sammaye, Commented Jun 25, 2013 at 16:22

numbers1311407 · Accepted Answer · 2013-06-25 17:51:03Z

8

Positional notation in aggregation seems to still be unsupported, check out this ticket.

As @Sammaye says you'd need to either unwind the array first, or replace your coordinates array with an embedded lng/lat embedded doc, which would make this trivial.

Given the array structure, you might unwind and project the lat/lng like this:

myColl.aggregate([
 // unwind the coordinates into separate docs
 {$unwind: "$myCoordinates"},

 // group back into single docs, projecting the first and last
 // coordinates as lng and lat, respectively
 {$group: {
   _id: "$_id",
   lng: {$first: "$myCoordinates"},
   lat: {$last: "$myCoordinates"}
 }},

 // then group as normal for the averaging
 {$group: {
   _id: 0,
   lngAvg: {$avg: "$lng"},
   latAvg: {$avg: "$lat"}
 }}
]);

answered Jun 25, 2013 at 17:51

numbers1311407

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2 Comments

Doc_1faux Over a year ago

Thanks for your help, I could adjust my query with yours and it works! :) Unfortunately, I couldn't use a document for the lng/lat couple since I use a 2d geo index on it. I was worried about loosing any reference to the root document by using the unwind operator on the array but it seems it returns a copy of the root document for each value contained in the array by replacing the array by each value. I hope they'll fix positional notation in aggregation soon to simplify aggregation operations on 2d geo index.

numbers1311407 Over a year ago

According to the documentation, geospatial indexing should already work on lng/lat embedded docs.

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