How to add together a subset of elements of an array in MongoDB aggregation?

To add together a subset of elements of an array, use $first along with $sum. Let us create a collection with documents −

> db.demo610.insertOne({Values:[10,20,30,40,50]});{
   "acknowledged" : true, "insertedId" : ObjectId("5e9747b8f57d0dc0b182d62e")
}

Display all documents from a collection with the help of find() method −

> db.demo610.find().pretty()

This will produce the following output −

{
   "_id" : ObjectId("5e9747b8f57d0dc0b182d62e"),
   "Values" : [
      10,
      20,
      30,
      40,
      50
   ]
}

Here is the query to add together a subset of elements of an array in MongoDB aggregation −

> db.demo610.aggregate([
...    {$unwind:"$Values"},
...    {$group:{"_id":"$_id",
...       "1st":{$first:"$Values"},
...       "All":{$sum:"$Values"}}},
...    {$project:{"_id":"$_id",
...       "SumOfAllMinus1":{$subtract:["$All","$1st"]}}},
...    {$group:{"_id":null,
...       "SumOfAllExcept1stValue":{$sum:"$SumOfAllMinus1"}}}
... ])

This will produce the following output −

{ "_id" : null, "SumOfAllExcept1stValue" : 140 }