Ruby on Rails® for Microsoft Developers

4.7. Metaprogramming

Metaprogramming is defined as the act of programming code that is able to manipulate itself (or other code). A practical example of this showed up a few sections ago when you used methods such as attr_accessor to dynamically obtain getter and setter methods for instance variables. The ability to reopen classes, define classes and methods conditionally, and call methods that get defined when first invoked, are all good examples of Ruby's metaprogramming abilities.

Metaprogramming is generally encouraged within the Ruby community partially because it's particularly useful when defining Domain Specific Languages (DSLs). Most Ruby developers would agree that powerful metaprogramming techniques should not be abused when writing code that may end up becoming "too clever for its own good," but that a decent mastery of the subject can really separate a beginner from a pro Ruby programmer.

Rails makes extensive use of metaprogramming in its implementation, and you are encouraged to explore the topic further on your own once you've gained more confidence with Ruby and Rails. Even as a Rails developer (as opposed to a Rails implementer), metaprogramming can be very useful, particularly when you want to start creating and publishing your own plugins. It's important to provide you with one quick example though, in order to understand how ActiveRecord's dynamic finders work. If you have a User model with a user_name and a password attribute, in ActiveRecord you could issue ...