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Head First Rails by David Griffiths

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Chapter 5. Validating your Data: Preventing mistakes

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Everyone makes mistakes... but many of them are preventable! Even with the very best of intentions, your users will still enter bad data into your web app, leaving you to deal with the consequences. But just imagine if there was some way of preventing mistakes from happening in the first place. That’s where validators come in. Keep reading, and we’ll show you how to add clever Rails validation to your web app so that you can take control of what data is allowed in—and what needs to be kept out.

Watch out—there’s bad data in the room

Everything seemed to be going so well with the personal trainers’ web app, at least until the body builders showed up. The body builders say they’ve paid their gym dues, and have the receipts to prove it, but their payments aren’t showing up on the system.

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So what went wrong?

Clicking on the save button should have saved the data into the database, but something went wrong. Instead of saving the amount paid as $50, the amount got saved as 0.0. But how? Let’s take a look at the chain of events that occurred.

  1. A trainer enters “$50” into the paid field in the view, and then clicks on the save button. A value of “$50” is passed to the controller.

  2. The controller receives a value of “$50” for the amount paid, and ...

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