Postgresql – the way to paginate last updated rows from multiple tables

I want to have something like a newsfeed from multiple objects sort by most_recent. However I need to paginate (load more/offset) the results.

• Row1 = Post's – text, username
• Row2 = Photo's – text, picture_url, username
• Row3 = Post's – text, username
• Row4 = Photo's – text, picture_url, username

I was thinking having a table inheritance called Activity that will hold all types of objects, referencing them with the columns item_type and item_id.

This way I query directly from the Activity table.

This should work. But how to maintain the activity to "stay tuned" of it's items "updated_at"?

When I comment in a post, should the comment update the post "updated_at" column and the post update the "updated_at" column on the activity?

Thanks!

EDIT
Activities table:

  id   event      item_type   item_id   parent_type    parent_id    updated_at
  1   "create"      Post         1         Group           100
  2   "create"      Post         2         Group           100
  3   "create"     Comment       1         Post            1
  4   "create"      Photo        1         Group           100

Query: The latest root events (parent_type is "Group") from activities. Should bring Post 1, Photo 1 and Post 2.

SELECT * FROM activities WHERE
parent_type = "**Group**" and parent_id = 100
ORDER BY created_at

Still not retrieving the latest. Because the comments didn't affected the Post activity.