Mongodb sum the size of array fields

Question

so i have a bunch of simple documents like

{

  \"foos\": [
    ObjectId(\"5105862f2b5e30877c685c58\"),
    ObjectId(\"5105862f2b5e30877c685c57\"),
    Obje         


        

Answer 1

I know this is an old question but you can bypass project altogether if you want, so how about this?

db.profil.aggregate([
{
    "$match":{ "typ": "Organisation" }
},
{
    "$group":
    {
        "_id": null,
        "count":
        {
            "$sum": { "$size": "$foos" }
        }
    }
}])

The output remains the same and it seems it's (slightly) faster.