Sunday, July 15, 2007

Abuse of method_missing?

Am I the only one who thinks the following DSL-ey trickery is an abuse of method_missing?

Here's the creation of some named routes.

ActionController::Routing::Routes.draw do |map|
  map.home '', :controller => 'main', :action => 'start'
  map.user_page 'users/:user', :controller => 'users', :action => 'show'

You call arbitrary methods on the map object, and that creates a route whose name is the method you called.

Here's the declaration of an ActiveRecord model's attributes using Hobo's new migration-generating style.

class User < ActiveRecord::Base
  fields do
    name :string, :null => false
    email :string
    about :text, :default => "No description available"

I haven't looked into the code, but I assume the block is instance_eval'ed against some object whose method_missing builds up attribute meta-data where the name of the missing method becomes the attribute name.

Introducing new symbols into your system by invoking them as methods on bizarre (sometimes hidden) objects strikes me as a nearly useless twisting of Ruby's flexibility. If you're working with something that's purely DSL-ish, that's one thing, but if we're talking about a tiny little internal DSL embedded in otherwise idiomatic Ruby code, and, needless to say, being edited by Ruby developers, this just seems to introduce confusion.

Of the two uses, I actually prefer Hobo's because it goes farther than Rails' away from idiomatic Ruby and towards a DSL. Since I actually see that I'm sending messages to map when creating a named route, it frustrates me that this object exposes its functionality through method_missing, and I'm therefore unable to look up the API reference in the normal way. What is that thing? Does it have any methods that might conflict with my route names? We know about connect. Hopefully that's the only one.[1]

Do you think this sort of use of method_missing is advisable? How have you used and abused it?

[1] Actually, a look at routing.rb shows that ActionController::Routing::Routes.draw yields an ActionController::Routing::RouteSet::Mapper, which also (in the neighborhood of line 1000[2]) defines named_route: not a likely name conflict, but arguably a clearer way to define your routes.

ActionController::Routing::Routes.draw do |map|
  map.named_route :home, '', :controller => 'main', :action => 'start'
  map.named_route :user_page, 'users/:user', :controller => 'users', :action => 'show'

[2] Yeah, line one-thousand. The Rails team are trained professionals: please don't try that at home.

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