entries_controller.rb

class EntriesController < ApplicationController
  def index
    @entries = scope
  end

  def show
    @entry = scope.first
  end

  def scope
    associations = Entry.associations.map {|name| name.to_s}
    params[:id].split('/').inject(nil) do |scope, id|
      if associations.include? id
        if (scope)
          scope.send(id)
        else
          id.singularize.titleize.constantize.dataset
        end
      else
        if scope && scope.model.name == 'Entry' && scope.opts[:where] && scope.opts[:where].op != :IN
          scope = scope.entries
        end
        (scope || Entry).where :entries__id => id
      end
    end
  end
end

entry.rb

# STI with M:M on itself through a "relations" table

class Entry < Sequel::Model
  plugin :single_table_inheritance, :kind
  many_to_many :entries,    :class => :Entry,     :join_table => :relations, :left_key => :subject_id, :right_key => :object_id
end


class Product < Entry

end

class Slideshow < Entry

end

# Join table
class Relation < Sequel::Model
  many_to_one :subject, :class => :Entry
  many_to_one :object, :class => :Entry
end

gistfile1.sql

-- 

-- 200, 1 product with id 8
-- /entries/5/6/7/products/8

SELECT `entries`.* 
FROM   `entries` 
WHERE  ( ( `entries`.`kind` IN ( 'Product' ) ) 
         AND ( `entries`.`id` IN (SELECT `relations`.`object_id` 
                                  FROM   `entries` 
                                         INNER JOIN `relations` 
                                                 ON ( `relations`.`subject_id` = 
                                                    `entries`.`id` ) 
                                  WHERE  ( ( `entries`.`id` IN (SELECT 
                                             `relations`.`object_id` 
                                                                FROM   `entries` 
                                             INNER JOIN `relations` 
                                                     ON ( 
                                             `relations`.`subject_id` = 
               `entries`.`id` ) 
               WHERE  ( ( `entries`.`id` IN (SELECT 
               `relations`.`object_id` 
                           FROM   `entries` 
               INNER JOIN `relations` 
                ON ( 
               `relations`.`subject_id` = 
               `entries`.`id` ) 
               WHERE  ( `entries`.`id` = '5' )) ) 
               AND ( `entries`.`id` = '6' ) )) ) 
               AND ( `entries`.`id` = '7' ) )) ) 
         AND ( `entries`.`id` = '8' ) ) 
LIMIT  1 

-- 200 /entries/5/6/7/entries/8 
-- 200 /entries/5/6/7/8 
-- 200 /5/6/7/8
-- 404 /entries/5/6/7/slideshows/8 
-- 404 /5/6/7/7

-- all slideshows related to all products
-- /products/slideshows
SELECT `entries`.* 
FROM   `entries` 
WHERE  ( ( `entries`.`kind` IN ( 'Product' ) ) 
         AND ( `entries`.`id` IN (SELECT `relations`.`object_id` 
                                  FROM   `entries` 
                                         INNER JOIN `relations` 
                                                 ON ( `relations`.`subject_id` = 
                                                    `entries`.`id` )) ) ) 
LIMIT  1 
-- all entries related to a product
-- /products/3/entries

SELECT `entries`.* 
FROM   `entries` 
WHERE  ( ( `entries`.`kind` IN ( 'Product' ) ) 
         AND ( `entries`.`id` IN (SELECT `relations`.`object_id` 
                                  FROM   `entries` 
                                         INNER JOIN `relations` 
                                                 ON ( `relations`.`subject_id` = 
                                                    `entries`.`id` ) 
                                  WHERE  ( `entries`.`id` = '3' )) ) ) 
LIMIT  1