How to return ActiveRecord relation of nested resources

Question

i have an Article class

class Article < ActiveRecord::Base
    belongs_to :category
end

and Category class

class Category < ActiveRecord::Base
    has_many :articles
    has_many :categories
    belongs_to :category
end

Soon I'm gonna use Thinking Sphinx search engine on this Article model. But i want it to be used only on articles with specific category. Each category has many subcategories. in other words, it has many subcategories. and those subcategories MIGHT(!) also have subcategories. It may look like this:

Politics - World
         - Local

Sport - Football
      - Boxing - Amateur
               - Professional

So if a User want to search through Politics, both Local and World should be on the table, when he wants Sport, we go through Football, and Amateur and Professional boxing. My question is how can i write a method/scope to return all articles with categories that are 'under' chosen category? Remember that i intend to use ThinkingSphinx later, so it cant be nice and easy array, i need ActiveRecord Relation.

Answer 1

You can recursively get the category and subcategory IDs into a single array, then use an "IN" statement in the "where" clause, like so:

class Category < ActiveRecord::Base

  ...

  def all_subcategory_ids
    cat_ids = []
    cat_ids << self.id
    self.categories.each do |category|
      cat_ids << category.id
      cat_ids << category.all_subcategory_ids
    end
    #since we have a mixed array of ids and sub-arrays, we need to flatten
    #the cat_ids array so it is just a one-dimensional array of ids [Ruby Array#flatten](http://ruby-doc.org/core-2.0.0/Array.html#method-i-flatten) 
    cat_ids.flatten
  end
end  

And then in your Article model, your scope would look like this:

class Article < ActiveRecord::Base

  ...

  def self.in_category_and_subcategories(category)
    Article.where("category_id IN (?)", category.all_subcategory_ids)
  end
end