The most important thing is to keep yourself focused. Rails is huge and complicated, you’ll always have to know what you want to do.

According to DHH a Rails application does not need too much documentation. (check the post here)

Testing controllers let me understand what are mocks and stubs.

Testing has_many, and habtm relationships with RSpec is now DRY.

The first, and the “good enough” reason to test your Ruby on Rails application thoroughly is that Ruby is not a compiler but an interpreter, if not tested your application can halt even on a banal syntax error.

Howto install and use RSpec.

You’ll have to document your app. First you’ll have to describe what it should do (via RSpec), and then how it does (using doc & diagram generator plugins).