update(...args) {
    invokeMap(this.models, "update", ...args);

    return this;
  }
Updates each model in the collection, and immediately persists all changes to the db.

     ```js
     let posts = user.blogPosts;

     posts.update('published', true); // the db was updated for all posts
     ```

     @method update
     @param key
     @param val
     @return this
     @public