Creating an activity feed with rails, AR and observers

23 May 2010

I recently had to build an activity feed at letitcast.com.
It’s a common thing to do, but I thought I might mention how I did it, and see if anyone else has suggestions/improvements.

We wanted to display to our actors new parts that correspond to their profile, new feedbacks and views of their auditions , recent comments …

So, I first created a model “activity”, here’s the migration :

  class CreateActivities < ActiveRecord::Migration
    def self.up
      create_table :activities do |t|
        t.integer :user_id, :null => false
        t.integer :activity_type, :null => false
        t.integer :target_id, :null => false
        t.string  :target_type, :null => false
        t.timestamps
      end

       add_index :activities, [:target_id, :target_type]
       add_index :activities, :user_id

    end

    def self.down
      drop_table :activities
    end
  end

As you can see it’s a polymorphic model, so the activity can be related to any active record model.

Here’s the model :

  class Activity < ActiveRecord::Base
    belongs_to :user
    belongs_to :target, :polymorphic => true
  
    default_scope :order => 'activities.created_at DESC', :limit => 10
  
    AUDITION_POSTED           = 1 #target --> model audition
    AUDITION_FEEDBACK_UPDATED = 2 #target --> model audition
    AUDITION_VIEWED           = 3 #target --> model video
    NEW_PART                  = 4 #target --> model part
  
    class << self
      def add(user, activity_type, target)
        return false if user.blank? or activity_type.blank? or target.blank?
        activity = Activity.new(:user => user, :activity_type => activity_type, :target => target)
        activity.save!
      end
    end
  
  end

At the beginning the activity types were mapped in an other database but it was a bit overkill and was reducing the performance a lot so I just added them into the model (AUDITION_POSTED, AUDITION_FEEDBACK_UPDATED, …)

I decided to create the activities via observers, example :

  class AuditionObserver < ActiveRecord::Observer

    # ACTIVITY OBSERVER : new audition posted
    def after_create(audition)
      if audition.actor
        Activity.add(audition.actor.user, Activity::AUDITION_POSTED, audition)
      end
    end

  end

So now, I can add any event via a simple variable in the activity model and an observer.

It was working really well but there were some performance issues with some events that required a lot of inserts in the activity model.
I first decided to execute this observer in a delayed job but after some research I ended up with a fast way to insert a batch of activities, here’s the method :

 def batch_add(users, activity_type, target)
    return false if users.blank? or activity_type.blank? or target.blank?
    inserts = []
    users.each {|user| inserts.push "('#{user.id}', '#{activity_type}', '#{target.id}', '#{target.class}', UTC_TIMESTAMP())" }

    sql = "INSERT INTO activities (`user_id`, `activity_type`, `target_id`, `target_type`, `created_at`) VALUES #{inserts.join(", ")}"
    ActiveRecord::Base.connection.execute sql
  end

To retrieve the activities :

  @user.activities

By default you’ll get 10 activities order by most recents.
If you want to change this behavior, just change the default_scope.

We’re also in the process of grouping the user activities to display them in a nice view, so there will maybe be a part II :-)