Active Record Find Tip

October 7th, 2006

Jamis Buck has come roaring back into blogging since his switch to Mephisto. Yesterday, he released another nice tip on Helping ActiveRecord finders help you. He wanted to have a way to display a random record from the database. At first, he was going to create a new action, but knowing that the action would function primarily the same as show, he thought it better to extend the find method in Active Record like so:

class Thing < ActiveRecord::Base
  def self.find(*args)
    if args.first.to_s == "random" 
      ids = connection.select_all("SELECT id FROM things")
      super(ids[rand(ids.length)]["id"].to_i)
    else
      super
    end
  end
end

Now he could call Thing.find(‘random’) to get a random record from the database. This means his show action can display a random record simply by hitting the thing/random restful route (where random would be passed in as the id). You can read the full article on his blog.

Posted by jnunemaker Filed in Core

6 Responses to “Active Record Find Tip”

  1. Nick Howell Says:
    December 12th, 2006 at 01:14 PM

    Might want to check out alias_method_chain:

    class Thing < ActiveRecord::Base class << self def find_with_random(*args) if args.first.to_s == "random" ids = connection.select_all("SELECT id FROM things") find_without_random(ids[rand(ids.length)]["id"].to_i) else find_without_random(*args) end end alias_method_chain :find, :random end end
  2. Nick Howell Says:
    December 12th, 2006 at 01:16 PM

    Wow, that was awful. Sorry about the lack of line breaks; left out a <pre>.

    
  class Thing &lt; ActiveRecord::Base
    class &lt;&lt; self
      def find_with_random(*args)
        if args.first.to_s == "random" 
          ids = connection.select_all("SELECT id FROM things")
          find_without_random(ids[rand(ids.length)]["id"].to_i)
        else
          find_without_random(*args)
        end
      end
      alias_method_chain :find, :random
    end
  end

    Better?

  3. Nathan Says:
    February 2nd, 2007 at 01:42 AM

    Its a little more verbose, but wouldn’t this suffice?

    Thing.find(:first, :order => “rand()”)

  4. Balint Says:
    February 3rd, 2008 at 05:20 PM

    This is quite concise but unfortunately it is not database independent. It is rand() in MySQL and random() in PostgreSQL, for example.

  5. ken Says:
    February 20th, 2008 at 03:32 PM I don’t like any of these solutions.
    • select_all is horribly inefficient (you want to SELECT all ids from the db when you only need one record?!).
    • SQL doesn’t define a PRNG, so using RAND or RANDOM is not portable.
    • MySQL5, at least, doesn’t seem clever enough to realize that “ORDER BY RAND LIMIT 1” is the same as just taking a single random offset, so it takes 10x as long here.

    I think what you want is simply:

    Thing.find :first, :offset => rand(Thing.count)

  6. autologo Says:
    August 8th, 2008 at 04:34 AM

    Very good article! Ruby forever :)

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