Golang：MongoDB - 计算投影中嵌套数组的长度

Question

In MongoDB i like to count all length of nested arrays togehter. It is just one mongoDB document.

idForDB := "621101966rf42c24a8f41b87"
ctx, cancel := context.WithTimeout(context.Background(), time.Second * 20)
defer cancel()
options := options.FindOneOptions{
        Projection: bson.M{
            "test":          "$test",
            "count": bson.M{}, // count should placed here
        },
    }
result := clients.MongoClient.Database("somedb").Collection("somecollection").FindOne(ctx, bson.M{"_id": idForDB}, &options)

All objects under number are to be counted

{
   "test":"other",
   "list":[
      {
        "number":[
            {
                "value": "1"
            }, 
            {
                "value": "2"
            },
            {
                "value": "2"
            }
        ]
      },
      {
        "number":[
            {
                "value": "1"
            }, 
            {
                "value": "2"
            },
            {
                "value": "2"
            }
        ]
      },
      {
        "number":[
            {
                "value": "1"
            }, 
            {
                "value": "2"
            }
        ]
      }
   ]
}

The output for the "Number" field should be 8 because there are 8 documents under the "number" object.

Answer 1

Using mongoDB syntax, you can use $reduce for it:

db.collection.aggregate([
  {$project: {
      test: "$test",
      count: {
        $reduce: {
          input: "$list",
          initialValue: 0,
          in: {$add: ["$$value", {$size: "$$this.number"}]}
        }
      }
    }
  }
])

See how it works on the playground example