Ruby/Rails Collection to Collection

Question

I have a two tables joined with a join table - this is just pseudo code:

Library
Book
LibraryBooks

What I need to do is if i have the id of a l

Answer 1

One problem with

l.books.map{|b| b.libraries}.flatten.uniq

is that it will generate one SQL call for each book in l. A better approach (assuming I understand your schema) might be:

LibraryBook.find(:all, :conditions => ['book_id IN (?)', l.book_ids]).map(&:library_id).uniq