How do I write an ActiveRecord scope that returns the most recently created 2 records that belong to their parent?

Question

For example, say I have an Author class and a Book class. How do I write a scope for books that are the most recently created records belonging to their author?

I know that a scope for the most recently created books is

scope :most_recent, -> { order(created_at: :desc).limit(4) }

However, that returns the most recently created books by any author.

I'm looking for something like:

author 1: Plato

id | title | created_at 1 | Apology | 23 Jul 2018 2 | Phaedo | 24 Jul 2018 3 | Republic | 25 Jul 2018

author 2: Seneca

id | title | created_at 4 | Oedipus | 3 May 2018 5 | Agamemnon | 4 May 2018 6 | Hercules | 5 May 2018

to return Hercules, Agamemnon, Republic, and Phaedo

Answer 1

scope :most_recent, lambda { |*args|joins(:author).where(books:{:author_id=>args}).order('books.created_on DESC').limit(2) }

Write the above scope in your Book Model. To query for most recent books by any author call:

Book.most_recent(author_id)

author_id = Id of author/Author Model object for which you want the most recent books.

Answer 2

You can add it in the Book class like this:

scope: most_recent_by_author, ->(author_id) { where(author_id: author_id).order(created_at: :desc).limit(2) }

To query:

Book.most_recent_by_author(1)

Answer 3

ALl the above answers are not following the Rails conventions of single responsibility. Having the scope on the Book like you have is the only correct answer!

class Book
  scope :most_recent, -> { order(created_at: :desc).limit(4) }
end

And query like this:

# for author with ID=3
Author.find(3).books.most_recent