Kaminari pagination breaking with where clause on has_many association

Question

We have two models

Book has_many :reviews

Review belongs_to :book

A Review has a rating (from 1 to 5).

book1
  review1 - rating 5
  review2 - rating 5
  review3 - rating 4
book2 
  review4 - rating 5
book3
  review5 - rating 5

Let say I need to filter all books that have a rating of 5 I would do something like:

@books = Book.includes(:reviews)
             .where(reviews: { rating: 5 })
             .page(params[:page]).per(3)

The issue is that I get the following

book1
  review1 - rating 5
  review2 - rating 5
book2 
  review4 - rating 5

This is not working and is breaking pagination as the @books.total_count is 2 not 3 (which is the triggering pagination as we require per(3)). How would you fix it ?

I would expect the query to render 3 Books with all the reviews matching the where clause

book1
  review1 - rating 5
  review2 - rating 5
book2 
  review4 - rating 5
book3
  review5 - rating 5

Answer 1

Pagination can be fixed by using

@books = Book.includes(:reviews)
             .where(reviews: { rating: 5 })

@books = Kaminari.paginate_array(@books).page(params[:page]).per(4)

This works as expected but is not performant as there is no limit applied to @books on the first query. I think this should be optimized by using manual pagination.