Datasheet

Don’t Repeat Yourself (DRY)

Another important Rails principle is to avoid duplicating information. A tradi-

tional program contains code describing database tables. The code tells the

rest of the program about the structure of the tables. Only after this descrip-

tive code is in place can the rest of the program read data from the database.

But in some sense, the description of a database is redundant. A program can

examine a database and automatically deduce the structure of the database’s

tables. Any descriptive code simply repeats the obvious. “Hey, everyone.

There’s a gorilla in the room. And there’s an

image column in the photos

database table.” So what else is new?

In computer programming, repetition is bad. For one thing, repetition of infor-

mation can lead to errors. If the description of a database is inaccurate, the

program containing the description doesn’t work. (My HMO asks for my

address on every claim form. But my address hasn’t changed in the past ten

years. Occasionally, the folks who process the claim forms copy my address

incorrectly. They mail a reimbursement check to the wrong house. Then I make

ten phone calls to straighten things out. That’s a danger of having more than

one copy of a certain piece of information.)

Aside from the risk of error, the duplication of information means more

work for everyone. With traditional database programming, you must track

every decision carefully. If you add a column to a database table, you must

update the description of the database in the code. The updating can be

time-consuming, and it discourages agility. Also, if each change to a database

table requires you to dive into your code, you’re less likely to make changes.

If you avoid changes, you might not be responding to your customer’s ever-

changing needs.

Let’s Get Going

You can read this chapter’s lofty paragraphs until you develop a throbbing

headache. But the meaning behind these paragraphs might be somewhat elu-

sive. Do you feel different when you switch from C++ or Java to programming

in Ruby? Does Rails really speed up the development cycle? Can you create

an application in the time it takes to find a Starbucks in Manhattan? If you

find these questions intriguing, please read on.

Chapter 1: Welcome to the World of Ruby on Rails

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