For some collection with a field { wins: Number }, how could I use MongoDB Aggregation Framework to get the total number of wins across all documents in a collection?
If I have 3 documents with wins: 5, wins: 8, wins: 12 respectively, how could I use MongoDB Aggregation Framework to return the total number, i.e. total: 25.
To get the sum of a grouped field when using the Aggregation Framework of MongoDB, you'll need to use $group and $sum:
db.characters.aggregate([ {
$group: {
_id: null,
total: {
$sum: "$wins"
}
}
} ] )
In this case, if you want to get the sum of all of the wins, you need to refer to the field name using the $ syntax as $wins which just fetches the values of the wins field from the grouped documents and sums them together.
You can sum other values as well by passing in a specific value (as you'd done in your comment). If you had
{ "$sum" : 1 },
that would actually be a count of all of the wins, rather than a total.
[ ] ? It works without them, i.e. db.characters.aggregate({$group ... }) instead of db.characters.aggregate([{$group ... }]) .
_id: null is the key here. +1 you tried to use the aggregation framework without thinking about using _id: null.
$groupoperation as shown in the docs.db.characters.aggregate([{$group:{_id:'id',wins:{$sum:1}}}]);but without any luck. It returns how manywinsfields i have instead of values from the wins.db.characters.aggregate( [ { $group: { _id: null, total: { $sum: "$wins" } } } ] )