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Filtered Collections

This plugin is a alpha version and has not been tested exhaustively in production

The main purpose of this plugin is to solve the problem of having collections of objects which cost a lot to calculate (for example, the last posts of my friends, but only one per each friend, or the last activity of the users from Spain).

This is done by saving already filtered and calculated collections in a persistent storage (by the moment only Lightcloud with Tokyo Tyrant is supported) which is very fast to access.

Those collections are pluggables in your application, by defining them into the folder lib/collections: they are declared as new classes that have to inherit from FilteredCollections::Collection class.

By default a collection belongs to the objects of a class, i.e. an user. Every object that has a collection has a new method that refers to the collection.


This is not a generic plugin, but one with a lot of dependencies (opinions :D) and not easy to change:

The idea is to let you to choose different storage's (you are welcome to fork it ;)


script/plugin install git://

Also, you have to declare your lightcloud list of servers in config/lightcloud.yml.

Basic usage

After installing it a new initialiser will be copied into your config/initializers folder. That initialiser loads every file in lib/collections with extension *.rb and calls its method self.set_callbacks that we'll explain down.

As we said before, the idea is that the collections belong_to the objects of a class. For example, I want that a given user has a collection of the reviews created only by his friends. I can indicate this by adding:

class User < ActiveRecord::Base
  has_collection :friends_reviews, :belongs_to => ''

Automatically every user becomes the owner of one collection, and can access to it calling the method :friends_reviews. For example:

>> User.first.friends_reviews.find(:all)
=> #<FriendsReviews:0x31f2e8c @belongs_to=1, @elements=[{21119=>Sun, 16 Aug 2009 10:26:10 UTC +00:00}], @elements_ids=[21119], @elements_class=Review(id: integer, user_id: integer, film_id: integer), @locked=0, @order=:desc, @total_elements=1, @order_by_attribute=:updated_at>

Of course, it is necessary to have declared a collection class with name FriendsReviews in lib/collections folder. What's the structure of a collection?

Schema of a collection

class FriendsReviews < FilteredCollections::Collection

  def self.attributes
    { :elements_class => Review, :order_by_attribute => :updated_at, :order => :desc }

  def self.set_callbacks
    Review.send(:after_save, do |review|
        UserFriend.find(:all, :conditions => ["friend_id = ?", review.user_id], :select => "user_id").map(&:user_id).each do |user_id|
          eval("#{FriendsReviews.builder(:belongs_to => 'user_id')}.store_element( review )")


  def self.build_all
    User.find(:all, :select => "id").map(&:id).each { |user_id| user_id ) }

  def user )
    user_id = user.is_a?(User) ? : user
    UserFriend.find(:all, :conditions => ["user_id = ?", user_id], :select => "friend_id").map(&:friend).each do |friend|
      eval("#{self.builder(:belongs_to => 'user_id')}.store_elements( )")


This is a real example of collection. Let's analyse it!


This class method declares a Hash with a list of required keys. This keys are:

  • :elements_class: the class of the elements stored in the collection. There is one only type of objects in each collection

  • :order_by_attribute: the attribute of the elements with which they are going to be ordered by. It can be a method or an attribute

  • :order: takes two values :asc or :desc


This method defines a set of callbacks that are required to fill the collection.

In our example, every time an user saves a Review, for every one of his friends, their collections are updated with the new review.


Build the collection for every object that owns one.

Build the collection for one object.

Accessing the elements

The idea is that the way to access the elements of the collection is the same whatever the elements you store. You can get the elements of a collection in two ways:

  • find(:type, options = {}) method, where :type can be :all or :first and the options only can be :limit and :offset. The order is given by the collection and cannot be changed, but you can always get all elements and reorder with Ruby sort methods.

  • paginate( options = {} ) method, where the allowed options are :page and :per_page

Some examples:

@reviews = current_user.friends_reviews.find(:all)
@reviews = current_user.friends_reviews.paginate(:page => params[:page], :per_page => 50)


As the collections for your application depend on you, also the tests. We recommend you to write some unit tests for every collection, specially for testing the callbacks.

For example, we have our tests in a folder named test/unit/collections.

It is important to notice that the callbacks of the collections (loaded in the initialised) are not set in the test environment, in order to not influence in the rest of tests (if you run your tests a lot of callbacks will be executed while they are not necessary all the time). So you'll have to call the method set_callbacks inside your test file.

Acts as stored in cache

This is a small hack to improve the performance of the collections: internally, a collection stores the list of the identifiers of the elements. Methods find and paginate loads the object given its identifier with a simple ActiveRecord::Base.find.

We recommend you to use acts_as_stored_in_cache which stores every object in Memcached every time it changes. That way, when the collection is loaded, instead of a ActiveRecord::Base.find a read from Memcached is performed.

Some advices

If you define a lot of callbacks your application will become slower and slower. Be careful and use a queue system.


  • Improve the documentation with some examples

  • Let the storage system to be configurable

  • Allow to have elements of different classes

  • Allow transactions when big changes occurs

Copyright (c) 2009 Fernando Blat, released under the MIT license

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