Fully instantiate an ActiveRecord object, complete with ID, without touching the DB

magically_instantiated.rb

# We had a situation where the site subdomain object had to be read on every single request and were looking for a way to reduce the DB hits, this worked, although you have to be careful to expire the cache properly if the object in question changes

def current_site(memoize = true)
  # (assumes "subdomain" and "domain" are available from URL information, this code may need to be tweaked)
  @current_site = nil unless memoize
  @current_site ||= Site.allocate.init_with('attributes' =>
    Rails.cache.fetch("Site#{subdomain}.#{domain}") do
      Site.find_by_subdomain_and_domain(domain, subdomain).attributes
    end
  )
end