Streamed CSV Export using node-restify and mongoose

api.js

/**
 * # Some Demo API Service
 */

var restify = require('restify');
var map = require('map-stream');
var csvify = require('../helpers/csvify');

var Team = require("../models").Team;

module.exports = {
  /**
   * ## CSV Export
   */
  csvExport: function (req, res, next) {
    res.writeHead(200, {
      'Content-Type': 'text/csv',
      'Content-Disposition': 'attachment; filename=export.csv'
    });

    res.write(csvify.rowFromArray(Team.csvHeader()));
    
    Team
    .find({accepted: true})
    .populate({path: '_users'})
    .stream()
    .pipe(map(function teamToArray (team, callback) {
      callback(null, team.mapToCSV());
    }))
    .pipe(map(function myCSVify (data, callback) {
      callback(null, csvify.rowFromArray(data));
    }))
    .on('error', function csvError (err) {
      console.log("oh noes, csv error!", err.stack);
    })
    .pipe(res);
  }
};

csvify.js

/**
 * # CSV Helpers
 */

/**
 * @method CSV Escape
 * @param  {String} field
 * @return {String}
 */
function csvEscape (field) {
  return '"' + String(field || "").replace(/"/g, '""') + '"';
}

/**
 * @method Array to Single CSV Row
 * @param  {Array}  array  A list of fields
 * @return {String} One row of a CSV file
 */
function array2csvRow (array) {
  return array.map(csvEscape).join(',')+'\n';
}

/**
 * @method Array to CSV
 * @param  {Array} array  An array of arrays (containing fields)
 * @return {String}       CSV representation
 */
function array2csv (array) {
  return array.map(array2csvRow).join('');
}

module.exports = {
  escape: csvEscape,
  rowFromArray: array2csvRow,
  fromArray: array2csv
};

This was helpful for me too. Populate wasn't working for me, so I used cursor() instead of stream().
Team .find({accepted: true}) .populate({path: '_users'}) .cursor()

Thank you, this part was specially helpful:

res.writeHead(200, { 'Content-Type': 'text/csv', 'Content-Disposition': 'attachment; filename=export.csv' });