Rails query for associated model with max column value - sql

Question

I have 3 models in rails: Author, Book, and Page. pages belongs to book, books belong to author as so:
class Author < ActiveRecord::Base
has_many :books
end
class Book < ActiveRecord::Base
belongs_to :author
has_many :pages
end
class Page < ActiveRecord::Base
belongs_to :book
end
the Page model has a column called page_number. I'm using Postgres.
My question is this: Assuming a have an author #author, how do query for all that author's last pages? In other words, I want the last page of each book written by that author. I am trying the following which isn't working:
Page.where(book_id: #author.books.pluck(:id)).select('MAX(page_number), *').group(:book_id)
EDIT
The following 2 lines work, but I would love to learn of a faster/cleaner solution:
all_pages = Page.where(book: #author.books)
last_pages = all_pages.select{ |a| !all_pages.select{ |b| b.book_id == a.book_id}.any?{ |c| c.page_number > a.page_number } }

Answer 1

The most efficient way might be leveraging postgres' window functions
A query like this doesn't fit into the activerecord common use case, so you may have to use find_by_sql, but it may be well worth it.
In your case, grabbing the book ids first may be a good call, as joining or an additional subquery may not be advantageous—your call.
Let's say you have a list of book ids from #author.books.ids. The next thing we want is a list of pages "grouped by" book so we can pluck off the last page for each group. Let 1,2 be the book ids for the author in question.
We can use a window function and the rank function in postgres to create a resultset where pages are ranked over partitions (group) of book. We'll even sort those partitions of pages by the page number so that the maximum page number (last page) is at the top of each partition. The query would look like this:
select
*,
rank() over (
partition by book_id order by page_number desc
) as reverse_page_index
from pages
where book_id in (1,2)
Our imagined pages result set would look like this.
author 1, book 1, page 3, rank 1
author 1, book 1, page 2, rank 2
author 1, book 1, page 1, rank 3
author 1, book 2, page 6, rank 1
author 1, book 2, page 5, rank 2
author 1, book 2, page 4, rank 3
author 1, book 2, page 3, rank 4
author 1, book 2, page 2, rank 5
author 1, book 2, page 1, rank 6
The pages records are partitioned by book, sorted by page number ascending and given a rank amongst their partition.
If we then want only the first ranked (last page) for each book after the window calculation is performed, we can use a sub-select like so:
select *
from
(
select
*,
rank() over (
partition by book_id order by page_number desc
) as reverse_page_index
from pages
where book_id in (1,2)
) as pages
where reverse_page_index = 1;
Where we filter the above imagined result set to just page records where the rank (reverse_page_index) is 1 (i.e. the last page).
And now our result set would be:
author 1, book 1, page 3, rank 1
author 1, book 2, page 6, rank 1
You could also order this resultset by last modified or whatever you need.
Toss that query in a find_by_sql and you'll have some activerecord objects to use.