express-app.js

// I wanted a proof-of-concept for streaming binary data from a MongoDB document with Mongoose.
// I'm using express as web framework here but you should be able to pipe to anything that expects a stream.
// Based on https://gist.github.com/1403797

//// [ other express app stuff... ]

app.get('/files/:file', function(req, res, next) {
  var queryfileId = req.params.file;
  var GrabBinaryForField = require('./streamformatters').GrabBinaryForField;
  // "buffer" is the field on my model that stores the binary data
  return Asset.findById(queryfileId).select('buffer').stream().pipe(new GrabBinaryForField('buffer')).pipe(res);
});

streamformatters.js

var Stream = require('stream').Stream;

function GrabBinaryForField(field) {
  this.field = field;
  Stream.call(this);
  this.writable = true;
  this._done = false;
}

GrabBinaryForField.prototype.__proto__ = Stream.prototype;

GrabBinaryForField.prototype.write = function (doc) {
  this.emit('data', new Buffer(doc[this.field]));
  return true;
}

GrabBinaryForField.prototype.end =
GrabBinaryForField.prototype.destroy = function () {
  if (this._done) return;
  this._done = true;

  // done
  this.emit('end');
}

exports.GrabBinaryForField =
module.exports.GrabBinaryForField = GrabBinaryForField;

nice. probably don't need the extra Buffer wrap code: this.emit('data', doc[this.field])