Optimizing async joins for mongodb (mongoose) using async.js

I'm building a media viewer web app for a kiosk in node that uses data in mongo. To render the viewer, it gathers together all the Asset objects (representing video files), each of which belong to a Category, each of which belong to a Medium.

This is my first time using mongo and node together, so I decided to use async.js to organize the code, but I don't know if I'm doing things efficiently or not. Is there a better way to organize the data or perform the queries? Does this look remotely sane?

Additionally, I commented out some code I thought looked correct but would mysteriously overwrite the object simply by adding another key to it. What am I doing wrong?

models.js : mongoose models

var mongoose = require('mongoose');

/* Mediums... Film, Video, TV */
var mediumSchema = mongoose.Schema({
    name: String,
    id: String,
    description: String
});
var Medium = mongoose.model('mediums', mediumSchema);

/* Categories ... */
var categorySchema = mongoose.Schema({
    medium: String, // refers to medium.id, not medium._id
    name: String,
    shortName: String,
    years: String,
    description: String
});
var Category = mongoose.model('categories', categorySchema);

/* Assets */
var assetSchema = mongoose.Schema({
    title: String
,   year: Number
,   duration: Number
,   medium_info: String
,   copyright: String
,   extended_info: String
,   filename: String
,   category_id: mongoose.Schema.Types.ObjectId
});
var Asset = mongoose.model('assets', assetSchema);

exports.Medium = Medium;
exports.Category = Category;
exports.Asset = Asset;

viewer.js : the handler that retrieves all the data and renders the html

exports.index = function(req, res){
  res.render('index');
};

exports.viewer = function(req, res) {
  var models = require('./models');
  var async = require("async");
  var mongoose = require('mongoose');
  mongoose.connect('mongodb://localhost/test');

  var db = mongoose.connection;
  db.on('error', console.error.bind(console, 'connection error:'));
  db.once('open', function callback () {});

  // provide category object, returns assets in that category
  function getAssetsForCategory(category, callback) {
    console.log("Looking for assets for category:" + category.name);
    models.Asset.find({category_id: category._id}, function(err, assets) {
      callback(err, assets);
    });
  }
  // provide medium label ("FILM","VIDEO","TV") returns list of categories
  function getCategoriesForMedium(medium, callback) {
    console.log("finding categories for " + medium);
    models.Category.find({medium: medium}, function(err, categories) {
      callback(err, categories);
    });
  }

  data = {};
  async.each(
    ["FILM","VIDEO","TV"],
    function(mediumName, mediumLookupComplete) {

      var mediumResults = {};
      var categoryResults = {};
      var assetResults = {};
      async.waterfall(
        [
          function(findMediumComplete) {
            // get medium object for its additional metadata
            models.Medium.findOne({id: mediumName}, findMediumComplete)
          },

          function(medium, findCategoriesComplete) {
            mediumResults.name = medium.name;
            mediumResults.description = medium.description;
            getCategoriesForMedium(medium.id, findCategoriesComplete);
          },

          function(categories, assetFetchComplete) {
            async.each(
              categories,
              function(category, categoryAssetLookupDone) {
                categoryResults[category._id] = category;
                /* I originally wanted to just insert the list of assets
                   into their appropriate category object here, but doing so
                   would overwrite the entire object--explanations? */
                // categoryResults[category._id].assets = [];
                getAssetsForCategory(category, function(err, assets) {
                  /* instead I have to use a separate temp object to store the
                     asset results, rather than just saying
                     categoryResults[category._id].assets = assets; */
                  assetResults[category._id] = assets;
                  categoryAssetLookupDone();
                });
              },
              assetFetchComplete
            );
          }
        ],
        function(err) {
          console.log("asset fetch complete");
          data[mediumName] = {};
          data[mediumName]["categories"]  = categoryResults;
          data[mediumName]["description"] = mediumResults.description;
          data[mediumName]["name"] = mediumResults.name;

          data[mediumName].categories = [];
          for (var catId in assetResults) {
            data[mediumName].categories.push({info: categoryResults[catId], assets: assetResults[catId]});
            //data[mediumName]["categories"][catId] = {info: data[mediumName].categories[catId], assets: assetResults[catId]};
          }
          //data[mediumName]["assets"] = assetResults;
          //{medium: mediumResults, ass: assetResults};
          mediumLookupComplete(err);
        }
      );
    },
    function(err) {
      mongoose.connection.close();
      //res.send({data:data});
      res.render('viewer', {data:data});
    }
  );
};