A diagram of how the different tests fit together.

Use multiple kinds of tests to get good coverage across an app efficiently. Unit tests are focussed and fast, but slower integration tests will cover the communication between different parts of the system.
See an up-to-date version of this diagram on Google Drive here: https://docs.google.com/drawings/d/1LkyOA1z1Faj0lHbp4UpcRG2HM71TyDWwSQrun4uffcE/edit?usp=sharing
Add some notes for the methods Rails provides.