As you’ll see throughout this chapter, Rails expects all data validation to happen in the model layer and provides tools that make it easy to do there. If you find yourself putting data-checking code into the views or the controllers, pause for a moment—you’re quite likely doing something wrong.

The one probable exception is if you’re adding warnings for users working in your forms, avoiding a round trip to the server, but you should never rely on those to limit your data to the correct types. All that work should do is give users more information more rapidly.