How to sort items from a newsfeed as last_active_at?

Question

In this question I've got two tables for the database:

• Post (has_many :comments)
• ActivityFeed (belongs_to item, :polymorphic)
• Comment (belongs_to :post)

Suppose that inside a group in facebook happen these sequential actions:

1. User A posted a status "Status 1"
2. User B posted a status "Status 2"

and then each one of these 2 posts has triggered the creation of a FeedItem (Activity) for the group.

Now when the user loads the page, the feed activities are sorted by created_at/updated_at. The feed activities are a polymorphic set of objects (can be post, photo, poll, file...).

If in a second moment User A comments the post of user B, the correct order for the feed should now be:

1. "Status 2"
2. "Status 1"

So what should happen when User A comments on a "Status 2"? Should I have to find the related Post (where the comment is being added) and update the timestamp from the activity that the Post is related to?

I need some advices for this. Thanks!

---

UPDATE

class Comment < ActiveRecord:Base
  belongs_to :post, touch: true
end

class Post < ActiveRecord:Base
  has_many :comments
  act_as_activity -> on_create will create a Activity representing this object
end

class Activity < ActiveRecord:Base
  belongs_to :item, :polymorphic => true
  belongs_to :parent, :polymorphic => true
  belongs_to :user
end

So the feed query will lookup for the activities table, since I can have multiple types of objects in the feed. And the timestamp must be refresh in this table when comment a post, because the query is done in Activity table:

SELECT * FROM Activity 
              ORDER BY updated_at

Answer 1

Using bdares' idea of using an after_create callback it might look something like this.

class Comment
  belongs_to :post, touch: true
  after_create do 
     self.post.activity.last_update_at = Time.now
     self.post.activity.save!
  end
end

This means that you're sorting Activity by that last_update_at field.