Add a .findOrBuild() method to Backbone Models

gistfile1.js

  // Finding and creating models
  // ===========================
  // 
  // We might need to get a reference to a model from anywhere in our code
  // and we want to make sure that these references all point to the same model.
  // Otherwise updates to one copy of the model won't affect another.
  //
  // The methods here let you get models through a wrapper that will either
  // create the model if it doesn't exist anywhere, or return a reference to
  // the existing model. E.g.
  //
  //     var Train  = ModelWithFind.extend();
  //     var thomas = Train.findOrBuild("thomas");
  //     thomas == Train.findOrBuild("thomas"); // true
  //     thomas == new Train({ "id" : "thomas" }); // false
  // 
  // Models are looked up by their `id`. If the model doesn't exist yet then it
  // is created and the attribute given by `idAttribute` is set to the passed value.
  var ModelWithFind = Backbone.Model.extend({}, {
    // When trying to get a reference to a model, you should use `findOrBuild`.
    // This takes one argument which is the model's `id` and will either return the
    // existing model with that `id` or create it if it doesn't exist.
    findOrBuild : function(id) {
      var model = this.find(id);
      if (model) return model;

      var options = {};
      options[this.prototype.idAttribute] = id;
      return this.build(options);
    },

    // `build` is a wrapper for the usual `new Model()`. It instantiates the model
    // but then saves it in the index of loaded models.
    build : function(options) {
      var model = new this(options);

      this.loadedModels = this.loadedModels || {};
      this.loadedModels[model.id] = model;

      // It's possible for a model's id to change. If it does we update our index.
      var self = this;
      model.on("change:" + model.idAttribute, function() {
        self.loadedModels[model.id] = self.loadedModels[model.previous(model.idAttribute)]
        delete self.loadedModels[model.previous(model.idAttribute)];
      })

      return model;
    },

    // `find` looks up a model in the index and returns it if it's present.
    find : function(id) {
      this.loadedModels = this.loadedModels || {};
      return this.loadedModels[id];
    }
  });