// This is a contrived example to demonstrate
// storing data on one side of a relationship

var PostSchema = new mongoose.Schema({
  title: String,
  slug: String,
  contents: String,
  categories: [{ type: mongoose.Schema.Types.ObjectId, ref: 'Category' }]
});

var Post = mongoose.model('Post', PostSchema);

var CategorySchema = new mongoose.Schema({
  name: String
});

var Category = mongoose.model('Category', CategorySchema);

// easy to get the categories for a post...

Post.find().populate('categories').exec(function(err, posts) {
  // you have posts with categories
});

// but harder to get the posts for a category.

Category.find().populate('posts').exec(function(err, categories) {
  // handle err
  async.forEach(categories, function(category, done) {
    Post.find().where('categories').in([category.id]).exec(function(err, posts) {
      category.posts = posts;
      done(err);
    });
  }, function(err) {
    // ... you have categories with posts
  });
});