Calculating collection stats for a subset of documents in MongoDB

This may not be the most efficient or accurate way to do it, but I ended up using a Mongoose plugin to get the size of the JSON representation of the document before it's saved:

module.exports = exports = function defaultPlugin(schema, options){    schema.add({        userId: { type: mongoose.Schema.Types.ObjectId, ref: "User", required: true },        recordSize: Number    });    schema.pre('save', function(next) {        this.recordSize = JSON.stringify(this).length;        next();    });}

This will convert the schema object to a JSON representation, get it's length, then store the size in the document itself. I understand that this will actually add a tiny bit of extra storage to record the size, but it's the best I could come up with.

Then, to generate a storage report, I'm using a simple aggregate call to get the sum of all of the recordSize values in the collection, filtered by userId:

mongoose.model('YouCollectionName').aggregate([{    $match: {         userId: userId    }},{     $group: {        _id: null,        recordSize: { $sum: '$recordSize'},        recordCount: { $sum: 1 }    }}], function (err, results) {   //Do something with your results});