Question

I have a voting system with two models: Item(id, name) and Vote(id, item_id, user_id).

Here’s the code I have so far:

class Item < ActiveRecord::Base
  has_many :votes

  def self.most_popular
    items = Item.all #where can I optimize here?
    items.sort {|x,y| x.votes.length <=> y.votes.length}.first #so I don't need to do anything here?
  end
end

There’s a few things wrong with this, mainly that I retrieve all the Item records, THEN use Ruby to compute popularity. I am almost certain there is a simple solution to this, but I can’t quite put my finger on it.

I’d much rather gather records and run the calculations in the initial query. This way, I can add a simple :limit => 1 (or LIMIT 1) to the query.

Any help would be great–either rewrite in all ActiveRecord or even in raw SQl. The latter would actually give me a much clearer picture of the nature of the query I want to execute.

Answer 1

Group the votes by item id, order them by count and then take the item of the first one. In rails 3 the code for this is:

Vote.group(:item_id).order("count(*) DESC").first.item

In rails 2, this should work:

Vote.all(:order => "count(*) DESC", :group => :item_id).first.item