Finding related articles using Jaccard index and tags

## Article.rb
# This method finds related articles using Jaccard index (optimized for PostgreSQL).
# More info: http://en.wikipedia.org/wiki/Jaccard_index

class Article < ActiveRecord::Base

  def related(limit=10)
    Article.find_by_sql(%Q{
      SELECT
      	a.*,
      	( SELECT array_agg(t.name) FROM taggings tg, tags t
      		WHERE tg.taggable_id = a.id AND tg.tag_id = t.id
      	) as tags,
      	((SELECT COUNT(*) FROM
      	((SELECT t.name FROM taggings tg, tags t
      		WHERE tg.taggable_id = a.id AND tg.tag_id = t.id
      	) INTERSECT
      	(SELECT t_2.name FROM taggings tg_2, tags t_2
      		WHERE tg_2.taggable_id = #{self.id} AND tg_2.tag_id = t_2.id
      	)) as intersection
      	)::float /
      	(SELECT COUNT(*) FROM
      	((SELECT t.name FROM taggings tg, tags t
      	WHERE tg.taggable_id = a.id AND tg.tag_id = t.id
      	) UNION
      	(SELECT t_2.name FROM taggings tg_2, tags t_2
      		WHERE tg_2.taggable_id = #{self.id} AND tg_2.tag_id = t_2.id
      	)) as union_total
      	)::float) as score
      FROM
      	articles a
      WHERE
      	a.published = 1 AND
      	a.id != #{self.id}
      ORDER BY score DESC, a.created_at DESC
      LIMIT #{limit}
    })
  end

end