Rails query to order records by single association row column

Question

Say I have a books table that has id and name columns and a has_many book_ratings association. The book_ratings table has id, rating, rating_date, and book_id columns and belongs_to :book. I'm displaying the books in a table with 3 columns: the books name, the highest rating for the book, and the most recent rating for the book. I want each column sortable and I'm using the Pagy gem for pagination. I want to make a single call to each table to avoid N+1.

I made this query which includes the max_rating and sorts by max_rating:

@pagy, @books = pagy(
  Book.joins(:book_ratings)
      .select('books.*,MAX(rating) as max_rating')
      .order('max_rating')
      .group('books.id')
)

With each book record, I can call book.max_rating to display the max_rating in the table. I cannot think of a way to also include the rating with the latest date and sort the records by that rating. I know I can use MAX(rating_date) to get the date of the book_rating with the most recent date, but I need a way to include the rating value of the book_rating with the latest date and also be able to order the book records by those values.

Any ideas?

Answer 1

To get this to work, I ended up using a select method and then chaining two joins methods. Here's what I ended up using:

Book.select('books.*,t2.rating as latest_rating,t4.rating as highest_rating')
    .joins(
      "INNER JOIN (
         SELECT book_ratings.rating, book_ratings.book_id, book_ratings.rating_date
         FROM book_ratings
         JOIN (
           SELECT book_id, MAX(rating_date) as max_date
           FROM book_ratings GROUP BY book_id
         ) t1
         ON t1.book_id = book_ratings.book_id
         AND t1.max_date = book_ratings.rating_date
       ) t2
       ON t2.book_id = books.id"
    )
    .joins(
      "INNER JOIN (
         SELECT book_ratings.rating, book_ratings.book_id
         FROM book_ratings
         JOIN (
           SELECT book_id, MAX(rating) as max_rating
           FROM book_ratings
           GROUP BY book_id
         ) t3
         ON t3.book_id = book_ratings.book_id
         AND t3.max_rating = book_ratings.rating
       ) t4
       ON t4.book_id = books.id"
    )
    .distinct

Calling .to_sql on that query gives this SQL:

SELECT DISTINCT books.*,t2.rating as latest_rating,t4.rating as highest_rating
FROM "books"
INNER JOIN (
  SELECT book_ratings.rating, book_ratings.book_id, book_ratings.date
  FROM book_ratings
  JOIN (
    SELECT book_id, MAX(date) as max_date
    FROM book_ratings
    GROUP BY book_id
  ) t1
  ON t1.book_id = book_ratings.book_id
  AND t1.max_date = book_ratings.date
) t2
ON t2.book_id = books.id
INNER JOIN (
  SELECT book_ratings.rating, book_ratings.book_id
  FROM book_ratings
  JOIN (
    SELECT book_id, MAX(rating) as max_rating
    FROM book_ratings
    GROUP BY book_id
  ) t3
  ON t3.book_id = book_ratings.book_id
  AND t3.max_rating = book_ratings.rating
) t4
ON t4.book_id = books.id

When iterating through this collection, latest_rating and highest_rating can be called on each object and provides the value from the rating column for either. It can also be used with .order('latest_rating asc') (or desc) or .order('highest_rating asc') (or desc)