Question

This is a library system, people can borrow books here. And each book belongs to a category. We’d like to give people some suggestions according to what kind of books they borrowed most.

Here are four models:

class Person < AR
  has_many :borrows
end


class Borrow < AR
  belongs_to :person
  belongs_to :book
end

class Category < AR
  has_many :books
end

class Book < AR
  has_many :borrows
  belongs_to :category
end

And I wrote SQL to find the books

SELECT * FROM books WHERE category_id = 
  (SELECT category_id FROM books WHERE id IN
    (SELECT book_id FROM borrows WHERE person_id =10000)
  GROUP BY category_id ORDER BY count(*) DESC LIMIT 1)
AND id NOT IN
  (SELECT book_id FROM borrows WHERE person_id =10000)

This seems to be working, but I wonder how could I write the finder in the Rails way…

Answer 1

You can do following things, write following in person.rb

has_many :books, :through => :borrows
has_many :categories_of_books, :through => :books,  :source => :category

&

def suggested_books
 Book.where("category_id IN (?) AND id NOT IN (?)",  self.categories_of_books, self.books)
end

Though it results in more than 1 query, but its clean, you just have to do:

   @user.suggested_books