What does $sum:1 mean in Mongo

Question

I have a collection foo:

{ "_id" : ObjectId("5837199bcabfd020514c0bae"), "x" : 1 }
{ "_id" : ObjectId("583719a1cabfd020514c0baf"), "x" : 3 }
{ "_id" : ObjectId("583719a6cabfd020514c0bb0") }

I use this query:

db.foo.aggregate({$group:{_id:1, avg:{$avg:"$x"}, sum:{$sum:1}}})

Then I get a result:

{ "_id" : 1, "avg" : 2, "sum" : 3 }

What does {$sum:1} mean in this query?

Answer 1

From the official docs:

When used in the $group stage, $sum has the following syntax and returns the collective sum of all the numeric values that result from applying a specified expression to each document in a group of documents that share the same group by key:

{ $sum: < expression > }

Since in your example the expression is 1, it will aggregate a value of one for each document in the group, thus yielding the total number of documents per group.

Answer 2

Basically it will add up the value of expression for each row. In this case since the number of rows is 3 so it will be 1+1+1 =3 . For more details please check mongodb documentation https://docs.mongodb.com/v3.2/reference/operator/aggregation/sum/

For example if the query was:

db.foo.aggregate({$group:{_id:1, avg:{$avg:"$x"}, sum:{$sum:$x}}})

then the sum value would be 1+3=4