failure.rb

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

puts "first.comments.count: #{first.comments.count}"
puts "first.comments.model.count: #{first.comments.model.count}"
puts "first.comments.where(:index => false).count: #{first.comments.where(:index => false).count}"

output.txt

SELECT COUNT(*) FROM "comments" WHERE "comments"."post_id" = ?  [["post_id", 3]]
first.comments.count: 2

SELECT COUNT(*) FROM "comments"
first.comments.model.count: 2

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