How can I calculate the disk space (bytes) used by the documents returned from a mongoose query?

Question

The scenario is as follows:

• using Mongoose.js in Node.js
• perform a query, like Model.find().where('something', 'value').exec(callback)
• you receive an array of documents in the callback

How can I determine the size of those documents on disk?

Approximations are OK, for me at least.

Answer 1

You can do this by executing your query as a lean query (to get plain JavaScript objects instead of Mongoose document instances) and then dipping down into the BSON library that's part of the native MongoDB driver and calling calculateObjectSize:

var bson = mongoose.mongo.BSON;

Model.find().where('something', 'value').lean().exec(function(err, docs) {
    var docsBsonSize = bson.calculateObjectSize(docs));
});

It won't give you the size on disk, but it should be a good approximation of the size of BSON docs.