How to query another collection on callback of first query.

So I have a few mongoose schema's defined.

var partSchema = mongoose.Schema({
  name: String,
  system: String,
  team: Array,
  quantity: Number, 
});
var part = mongoose.model('part', partSchema);

var peopleSchema = mongoose.Schema({
  name: String, 
  pass: String, 
  role: String,
  parts: Array
});

var people = mongoose.model('people', peopleSchema);

And I want to send queries based on these schemas, and more, to the user. As such...

app.get('/', function(req, res){
   people.find(function(err, persons){
     if (err) return handleError(err);
     console.log(persons);
     parts.find(function(err, things){
       res.render('in', { user: req.session.user, people:persons, parts:things });
     });
   });
});

To remain asynchronous, I would have to nest these calls, right? And I cant nest these calls, because "parts" is no longer defined. I'm wondering if I could do this, and how would I do it? Sorry if it's a noob question, but I've looked this up for days, and haven't seemed to find a solution. Thanks!