Updating nested keywords only works when we save the object with the nested object populated with keywords

[sbland]

If we save a object without populating the nested objects we lose the nested keywords. The modified example below shows the issue.

  var mongoose = require('mongoose');
  var mongooseKeywords = require('mongoose-keywords');

  var GenreSchema = new mongoose.Schema({
    title: String
  });
  GenreSchema.plugin(mongooseKeywords, {paths: ['title']});

  var ArtistSchema = new mongoose.Schema({
    name: String,
    genre: {
      type: mongoose.Schema.ObjectId,
      ref: 'Genre'
    }
  });
  ArtistSchema.plugin(mongooseKeywords, {paths: ['name', 'genre']});

  var Genre = mongoose.model('Genre', GenreSchema);
  var genre = new Genre({title: 'Jrock'});
  console.info(genre.keywords); // ['jrock']

  var Artist = mongoose.model('Artist', ArtistSchema);
  var artist = new Artist({name: "L'arc~en~Ciel", genre: genre._id}); /* genre saved as object id */
  console.info(artist.keywords); // ['larc en ciel', 'jrock']