size method ignores "group by" on has_many

A has_many that uses the :group option, returns invalid results for the size method.

class Book < ActiveRecord::Base
  has_many :reviews
  has_many :reviewers, :through => :reviews, :group => "reviews.id"
end

class Review < ActiveRecord::Base
  belongs_to :book
  belongs_to :reviewer
end

class Reviewer < ActiveRecord::Base
  has_many :reviews
  has_many :books, :through => :reviews, :group => "books.id"
end

After create a bunch of reviews...

  alan = Reviewer.find_by_name("alan")
  alan.books.size # => 4
  alan.books.to_a.size # => 2

This is the SQL generated by the second line ignores the :group option:

SELECT count(*) AS count_all FROM "books" INNER JOIN reviews ON books.id = reviews.book_id WHERE (("reviews".reviewer_id = 1))

it should be more like the follow, no?

SELECT count(distinct books.id) AS count_all FROM "books" INNER JOIN reviews ON books.id = reviews.book_id WHERE (("reviews".reviewer_id = 1))

Attached are detailed steps to reproduce the bug.