rails3 scope for count of children in has_many relationship

Question

trying to do a scope in rails3.

:book has_many :chapters 

I want scope :long to return books with > 10 chapters.

How best to structure

Answer 1

This should get you going:

class Book
  scope :long, joins(:chapters).
                 select('books.id, count(chapters.id) as n_chapters').
                 group('books.id').
                 having('n_chapters > 10')
end

Does it help?