Rails get count of association through join table

Question

This question is an offshoot from HABTM associations in Rails : collecting and counting the categories of a model's children.

Given:

class Cate         


        

Answer 1

You can use chain select and group to aggregate the count of books for each category. Your books_per_category method may look like this:

def books_per_category
  categories.select('categories.id, categories.name, count(books.id) as count').group('categories.id, categories.name').map do |c|
    {
      name: c.name,
      count: c.count
    }
  end
end

This will produce the following SQL query:

SELECT categories.id, categories.name, count(books.id) as count 
  FROM "categories" 
  INNER JOIN "books_categories" ON "categories"."id" = "books_categories"."category_id" 
  INNER JOIN "books" ON "books_categories"."book_id" = "books"."id" 
  WHERE "books"."store_id" = 1 
  GROUP BY categories.id, categories.name