Mongoose sum and group model's field.

mongoose-sum-fields.js

/* fetch all users user_totaldocs and user_totalthings values and want to sum those variables.*/

var userSchema = mongoose.Schema({
  local : {
    ...
    ...
    user_id          : String,
    user_totaldocs   : Number,
    user_totalthings     : Number
    ....
  }
});
var User = mongoose.model('User', userSchema);

/*For example, to calculate sums (per user document) you can use the $add expression in a $project stage:*/
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: */
User.aggregate(
    { $group: {
        _id: null,
        total:       { $sum: { $add: ["$user_totaldocs", "$user_totalthings"] } },
        totaldocs:   { $sum: "$user_totaldocs" },
        totalthings: { $sum: "$user_totalthings" }
    }}
)
/*
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). 
  i.e. _id: user_id
*/