- Classes can be no longer than one hundred lines of code.
- Methods can be no longer than five lines of code.
- Pass no more than four parameters into a method. Hash options are parameters.
- Controllers can instantiate only one object. Therefore, views can only know about one instance variable and views
- When a call comes into your Rails controller, you can only instantiate one object to do whatever it is that needs to be done. And your view can only know about one instance variable.