"specific case" object that stands for "any object" : is it a good idea ?

Question

I got tired of doing (more or less) this in my rails controllers :

class ThingsController < ApplicationController
 def index
 @category = Category.find( params[:category_id] ) if params[:category_id].present?
 scope = @category ? @category.things : Thing.scoped
 @things = scope.order( :title )
 end
end

...wich also leads to a lot of conditionals in the template if @category is absent.

I had this idea (a variation on the NullObject pattern) :

class Category < ActiveRecord::Base
 def self.one_or_any( id )
 where( id: id ).first || any_category
 end
 def self.any_category
 @any_category ||= begin
 new( title: "Any category" ) do |generic|
 generic.instance_eval{ def things ; Thing.scoped end }
 generic.readonly!
 generic.freeze
 end
 end
 end
end

Which would in turn allow me to do :

class ThingsController < ApplicationController
 def index
 @category = Category.one_or_any( params[:category_id] )
 @things = @category.things.order( :title )
 end
end

... and to get rid of category-related conditionals in the template.

I Just wonder if this is a good idea, and if it has drawbacks i don't see yet. Thoughts ?

Note : Of course the actual implementation can be different (use a specific subclass of Category instead of extending an instance, for example - or even using a decorator).

Answer 1

I'd try to keep it as simple as possible. Instead of multiple in-line conditionals or a weird abstraction, I think a single full-fledged indented conditional makes things very clear. There are two scenarios, write two branches:

class ThingsController < ApplicationController
 def index
 if params[:category_id].present? 
 @category = Category.find(params[:category_id])
 @things = @category.things
 else
 @category = nil
 @things = Thing.scoped
 end
 end
end

• m_x: ok, but the question was more about the "generic object" idea. I'd like to avoid that @category = nil because it involves a view riddled with conditionals - if i had an object that quacks like a category, it would be better