The routes file can grow to be unmaintainable and messy. Learn to keep things organized by extracting out blocks of routes into their own files.

Mountable Engines are a great way to extract code into its own namespace and allow the code to be reused in other applications. Other popular gems that are mountable engines are Devise and Doorkeeper.

When displaying a count of records, this will generate extra SQL Queries. Learn how to reduce the number of SQL queries called with counter caching the number of associated records.

Extend your image upload functionality with JCrop. Learn to redirect the user to a crop page once they have uploaded their image and save versions of the cropped images.

Chosen is a jQuery plugin that makes long, unwieldy select boxes much more user-friendly. Learn how to use Chosen in a few different ways in this episode; Rails form builder and Simple Form.