bug_fix.rb

class Comment < ActiveRecord::Base
  belongs_to :post, :inverse_of=>:comments

  scope :not_index, -> {where(:index => false)}
end

first = Post.create!(:name => "First")
first.comments.create!(:index => true)
first.comments.create!(:index => false)

puts "first.comments.not_index.count: #{first.comments.not_index.count}"

output.txt

SELECT COUNT(*) FROM "comments" WHERE "comments"."post_id" = ? AND "comments"."index" = 'f'  [["post_id", 3]]
first.comments.not_index.count: 1