Introspection

The module that pays off everything

You've built the object model (24), closures (25), and the Enumerable/pattern layer (26). Now the reward: metaprogramming — code that inspects and writes code at runtime. This is what Rails is made of. has_many, validates, enum, find_by_email — every one is metaprogramming, and by the end of this module you'll build a working validates yourself. The theme: Rails isn't magic, it's Ruby.

In an interview, say: "Metaprogramming is writing code that operates on code at runtime — introspecting objects and defining methods on the fly. Rails' DSLs are all built from it, so understanding it turns 'magic' into mechanism I can debug."