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

Include the $group operator pipeline stage after the $project step as follows:

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

This will group all the input documents from the previous $project stage and applies the accumulator expression $sum on the fooos field within the group to get the total (using your last example):

This can also be done by-passing the $project pipeline as:

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

Output

/* 0 */
{
    "result" : [ 
        {
            "_id" : null,
            "count" : 24
        }
    ],
    "ok" : 1
}

Answer 2

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.