Count how many posts came out after the last post ActiveRecord

Question

I have the following question in ActiveRecord.

Model and Example Fields

Post [ title:string content:string color:string ]

Imagine that my model receives recurring posts, but sometimes a specific post comes with a "white" color.

Now imagine that a post with the color "white" came out about 30 minutes ago, and several posts came out with other colors in those 30 minutes.

The problem is that I can't imagine how to count how many posts came out after the last post with white color!

Answer 1

While both of the proposed solutions will work they require instantiating a Post object which is unnecessary in my opinion instead we can make this a single query like so

class Post < ApplicationRecord 

  scope :posts_since, ->(color) {
         where(arel_table[:created_at].gt(
                  select(arel_table[:created_at].maximum).where(color: color).arel))}
end 

Callable as

Post.posts_since(:white) 

Resulting Query

SELECT 
  posts.* 
FROM 
  posts 
WHERE
  created_at > (SELECT MAX(posts.created_at) FROM posts WHERE posts.color = 'white'))

To "count" you can just add count e.g. Post.posts_since(:white).count