Category.rb

class Category < ActiveRecord::Base
  has_many :subcategories, :dependent => :destroy
  has_many :users, :through => :subcategories
end

gistfile1.rb

# This..

Category.first.users

# ..generates this:

SELECT "users".* FROM "users" INNER JOIN "categorizations" ON "users"."id" = "categorizations"."user_id" INNER JOIN "subcategories" ON "categorizations"."subcategory_id" = "subcategories"."id" WHERE "subcategories"."category_id" = 1

# Is there something like this..

Category.find([1, 2]).users

# That would generate this:

SELECT "users".* FROM "users" INNER JOIN "categorizations" ON "users"."id" = "categorizations"."user_id" INNER JOIN "subcategories" ON "categorizations"."subcategory_id" = "subcategories"."id" WHERE "subcategories"."category_id" IN (1, 2)

# ..besides this: 

User.find_by_sql('SELECT "users".* FROM "users" INNER JOIN "categorizations" ON "users"."id" = "categorizations"."user_id" INNER JOIN "subcategories" ON "categorizations"."subcategory_id" = "subcategories"."id" WHERE "subcategories"."category_id" IN (1, 2)')

Subcategory.rb

class Subcategory < ActiveRecord::Base
  belongs_to :category
  has_many :categorizations
  has_many :users, :through => :categorizations
end

User.rb

class User < ActiveRecord::Base
  has_many :categorizations
  has_many :subcategories, :through => :categorizations
end