I transcribed an interview with DHH [0] in which he also explains his motivations for choosing Ruby (instead of PHP or Java, for example), and where he explains some design principles for Rails.

[0] http://www.transcribed-interview.com/dhh-rails-david-heineme...

"One example I always pull out is “what you gonna call the primary key in your tables?” When I was working with PHP and Java, every single shop, almost every single application, would have its own naming scheme. Some would say they have the ‘Products’ table and then they'd have ‘productid’, others would have ‘product_id’, some people would have ‘prod_id’ or ‘p_id’ or ‘P_id’, and every time somebody made a new design decision it meant configuration. You now have to tell your models, your objects, how they're going to talk to this database table. Because it needs to know what the hell you called the freaking primary key column, and it just doesn't matter! Who cares what the primary key column is called? It just doesn't matter. It's going to have zero impact on the usefulness of your application."

and

"‘Dont repeat yourself’ is all about not having the same intentions spread out in multiple places. Don't have one configuration. If you're calling something, let's again take the example of the primary key. If you're calling that for ‘id’ you shouldn't have to configure that in three different places that all have to work together and all have to be changed together. You should just pick one authoritative place to have that information stored. And then you can make changes from there. It also goes with the the whole Ruby idiom: we don't want those Java boilerplate ten line things: that's repeating yourself. If you have the same idiom, if you have the same intentions, that should really be exceedingly a short expression. And that goes up throughout the entire framework. Just keep one place to change those things, and keep the idioms very short."