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?

Answer 2

Ah - to answer my own question in the comment above, I had to put the count in the HAVING:

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

Answer 3

An alternative is to make a subquery. While the join is more correct (and possibly also leads to better performance), you may end up with strange results if you combine multiple scopes that try to do grouping. A subquery is much less intrusive. For this example, it would be something like:

class Book
  scope :with_chapters_count, -> {
    select('books.*').
    select('(select count(chapters.id) from chapters where chapters.book_id = books.id) as chapters_count')
  }
  scope :long, -> {
    with_chapters_count.where("chapters_count > 10")
  }
end

Answer 4

In rails 4.0 this version works. You have to count() in the having clause. It seems that having clause doesn't see 'as n_chapters'.