Improving the related_posts feature of jekyll

Now that I have converted TechTinkering over to Jekyll, I have come up against a bit of a problem with site.related_posts: The results are always just the latest posts, and are not filtered or ordered for relevance. I see that lots of people are struggling with a similar problem and have therefore decided to write a plugin which will improve it. Because the posts on this site make use of tags, I decided to match against those to assess relevance.

The Code

I was in two minds as to how to structure this plugin: whether to write a straight monkey patch, or whether to put it in a module and include it. I went with the latter as I have heard a lot of talk about this being the preferred route to ease debugging. However it doesn't seem quite right because I have had to force the removal of the old related_posts method to do so. If anyone has any suggestions on this, then please leave a comment here or via the GitHub repo.

At the time of writing, the related_posts.rb plugin file looks like this:

require 'jekyll/post'

module RelatedPosts

  # Used to remove #related_posts so that it can be overridden
  def self.included(klass)
    klass.class_eval do
      remove_method :related_posts
    end
  end

  # Calculate related posts.
  #
  # Returns [<Post>]
  def related_posts(posts)
    return [] unless posts.size > 1
    highest_freq = Jekyll::Post.tag_freq(posts).values.max
    related_scores = Hash.new(0)
    posts.each do |post|
      post.tags.each do |tag|
        if self.tags.include?(tag) && post != self
          cat_freq = Jekyll::Post.tag_freq(posts)[tag]
          related_scores[post] += (1+highest_freq-cat_freq)
        end
      end
    end

    Jekyll::Post.sort_related_posts(related_scores)
  end

  module ClassMethods
    # Calculate the frequency of each tag.
    #
    # Returns {tag => freq, tag => freq, ...}
    def tag_freq(posts)
      return @tag_freq if @tag_freq
      @tag_freq = Hash.new(0)
      posts.each do |post|
        post.tags.each {|tag| @tag_freq[tag] += 1}
      end
      @tag_freq
    end

    # Sort the related posts in order of their score and date
    # and return just the posts
    def sort_related_posts(related_scores)
      related_scores.sort do |a,b|
        if a[1] < b[1]
          1
        elsif a[1] > b[1]
          -1
        else
          b[0].date <=> a[0].date
        end
      end.collect {|post,freq| post}
    end
  end

end

module Jekyll
  class Post
    include RelatedPosts
    extend RelatedPosts::ClassMethods
  end
end

Installation

Please look at the README file for the latest instructions.

1. Download the latest version of the plugin from the related_posts-jekyll_plugin repository
2. Copy related_posts.rb to your _plugins directory

Using site.related_posts

The plugin replaces the functionality of site.related_posts so you can use it as follows:

{% for post in site.related_posts %}
  <a href="{{ post.url }}">{{ post.title }}</a><br />
{% endfor %}