Query and sum all with mongoose - node.js

Question

I want to fetch all users user_totaldocs and user_totalthings and want to sum those variables.
How can it's possible? Here is user schema:
var user_schema = mongoose.Schema({
local : {
...
...
user_id : String,
user_totaldocs : Number,
user_totalthings : Number
....
}
});

Answer 1

You can use the Aggregation Pipeline to add calculated fields to a result. There are some examples below using the mongo shell, but the syntax in Mongoose's Aggregate() helper is similar.
For example, to calculate sums (per user document) you can use the $add expression in a $project stage:
db.user.aggregate(
// Limit to relevant documents and potentially take advantage of an index
{ $match: {
user_id: "foo"
}},
{ $project: {
user_id: 1,
total: { $add: ["$user_totaldocs", "$user_totalthings"] }
}}
)
To calculate totals across multiple documents you need to use a $group stage with a $sum accumulator, for example:
db.user.aggregate(
{ $group: {
_id: null,
total: { $sum: { $add: ["$user_totaldocs", "$user_totalthings"] } },
totaldocs: { $sum: "$user_totaldocs" },
totalthings: { $sum: "$user_totalthings" }
}}
)
You may want only the one total field; I've added in totaldocs and totalthings as examples of calculating multiple fields.
A group _id of null will combine values from all documents passed to the $group stage, but you can also use other criteria here (such as grouping by user_id).

Answer 2

You can use aggregation framework provided by mongodb. For your case --
if you want to fetch sum of user_totaldocs and sum of user_totalthings across the collection (meaning for all users), do --
db.user_schemas.aggregate(
[
{
$group : {
user_id : null,
user_totaldocs: { $sum: "$user_totaldocs"}, // for your case use local.user_totaldocs
user_totalthings: { $sum: "$user_totalthings" }, // for your case use local.user_totalthings
count: { $sum: 1 } // for no. of documents count
}
}
])
To sum user_totaldocs and user_totalthings for particular user in a collection(assuming there are multiple document for a user), this will return sum for each user, DO --
db.user_schemas.aggregate(
[
{
$group : {
user_id : "$user_id",
user_totaldocs: { $sum: "$user_totaldocs"}, // for your case use local.user_totaldocs
user_totalthings: { $sum: "$user_totalthings" }, // for your case use local.user_totalthings
count: { $sum: 1 } // for no. of documents count
}
}
])
No need to provide individual user id.
For more info read:
1. http://docs.mongodb.org/manual/reference/operator/aggregation/group/#pipe._S_group
2. http://docs.mongodb.org/manual/core/aggregation/