Ruby on Rails

I wonder what is the best way to delete all the records of the association on a specified condition.

For example, you have User model that has many Post(s). How would keep all the recent User's Posts and delete all the last Posts when a Users have more than 25 ?

There is a solution like that:

def self.delete_old_posts
    old_posts = []
    users = includes(:post)
    users.each do |user|
      old_posts =+ user.posts.offset(25) unless user.posts.empty?
    end


    old_posts.delete_all
end

Any other ideas ? Thank you.