edit_route.js

App.ComposeRoute = Ember.Route.extend({
  model: function() {
    return this.ensureMessage();
  },

  createMessage: function() {
    var transaction = this.get('store').transaction(),
        message = transaction.createRecord(App.Message, {});
    return message;
  },

  ensureMessage: function() {
    var app = this.controllerFor('application'),
        modelProp = 'composingMessage',
        model = app.get(modelProp);
    if (model === undefined) {
      model = this.createMessage();
      app.set(modelProp, model);
    }
    return model;
  }
}