mongodb nodejs - converting circular structure

I have some code that pulls all documents from a collection and puts it onto a webpage. a simplified version looks like this:

var mongodb = require("mongodb"),
    express = require("express"),
    mongoServer = new mongodb.Server('localhost', 27017),
    dbConnector = new mongodb.Db('systemMonitor', mongoServer),
    db;

var app = new express();

app.get('/drives', function(req, res) {
  db.collection('driveInfo', function(err, collection) {
    if (err) throw err;
    collection.find({}, function(err, documents) {
      res.send(documents);
    });
  });
});

dbConnector.open(function(err, opendb) {
  if (err) throw err;
  db = opendb;
  app.listen(80);
});

I have a driveInfo collection which contains a long list of documents. Each document contains nested objects. What I would like to do, is whenever someone visits /drives in their browser, to print the entire collection as a json object so that I can grab everything with jquery later (beginnings of an api)

However, I get an error saying "TypeError: Converting circular structure to JSON". The error on the page points to this line of code:

collection.find({}, function(err, documents) {
  res.send(documents);
});

I'm unsure what the problem is, or where the self-reference is. Am I not querying the collection properly?