create_activities.rb

Activity.record_timestamps = false

Post.all.each do |post|
  if post.published?
    post.create_activity(:updated_at => post.updated_at)
  end
end

Comment.all.each do |comment|
  obj = comment.commentable

  if !obj.respond_to?(:published?) or obj.published?
    if obj.activity
      obj.activity.update_attributes(:updated_at => comment.created_at)
    else
      obj.create_activity(:updated_at => comment.created_at)
    end
  end
end

Activity.record_timestamps = true