Robot Has No Heart

Previous efforts to find related posts with the classifier gem yielded no fruit, so I tried another approach using sphinx. Turned out to be a winner.

The basic theory is to index all posts by tag, then to find related posts just use the current post’s tags as a search string. Remember to exclude the current post from the search results. For this blog, I use tags for the main categories, which were corrupting the results – most everything is tagged ‘Ruby’ so it doesn’t add any value in determining likeness. So rather than indexing all tags I excluded some of the main ones.

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class Post < ActiveRecord::Base
  has_many :searchable_tags, 
           :through    => :taggings,
           :source     => :tag,
           :conditions => "tags.name NOT IN ('Ruby', 'Code', 'Life')"
  
  def related_posts(number = 3)
    Post.search(:limit => number + 1, :conditions => {
      :tag_list => tag_list.join("|")
    }).reject {|x| x == self }.first(number)
  end

  define_index do
    indexes searchable_tags(:name), :as => :tag_list
    # If you want to use this for normal search as well you'll have to 
    # add in title/body here as well
  end
end

For a more complete example, see the relevant RHNH commits: cdc0bf and d4d844

Showing links to related content is a good way to stop the bottom of your page from being a ‘dead end’. In the event that no related posts are found, I’m linking to the archives instead.