Question

I have 2 models with a many-to-many relationship (simplified here with books-authors example):

class Book < ActiveRecord::Base  
    has_and_belongs_to_many :authors
end

class Author < ActiveRecord::Base
    has_and_belongs_to_many :books
end

I need to display a list of books, and under each one a list of its authors.
Users are able to filter via a certain author.

I’m trying to filter via an author in the following way:
Let’s assume there’s only one resulting book, and it has 3 authors.

books = Book.find(:all, :include => :authors, :conditions => "authors.id = 5")

When I try to list the different authors of the books retrieved, I get only the author with ID=5.

books.first.authors.size   # => 1
books.first.authors.first.id   # => 5

I would expect rails to go and retrieve all the different authors associated with the books, either during the find query or as a follow-up query, yet it does not.

How can I solve this problem?
Is there something wrong with the associations? the find query?

Thanks.

Answer 1

This happens because you use :include in your find method. When you do that, you tell Rails to cache the associated result (eager loading) to avoid multiple database queries. And since the conditions you have specified only includes one of the three authors, that is the only one that will be cached and then looped through when you call .authors

I think the easiest solution would be to change :include to :joins. It will perform the same database query, but it will not cache the associated authors. So when you call authors for each book, it will perform a new database call to retrieve all authors for that book. As I said, the easiest but perhaps not the best since it could potentially result in a lot of queries to the database.

Here is another way you could do it:

book_ids = Book.all(:joins => :authors, 
                    :conditions => ["authors.id = ?", 5]).map(&:id)
books = Book.all(:include => :authors, 
                 :conditions => ["books.id IN (?)", book_ids])

This will first collect all id’s of the books related to a specific author. Then it will query the database again with the resulting book_ids and this time include all authors related to those books and cache it to avoid unnecessary db calls.

Edit:

If you really want to use only one call, then I think the only options is to write a little more SQL in the call. Perhaps like this:

books = Book.all(:include => :authors, 
                 :conditions => ["books.id IN (SELECT book_id FROM authors_books WHERE author_id = ?)", 5])