あれっ？降順ってASC？DESC？そんな時は、reverse_orderを使ってみましょう！ ref: http://qiita.com/lasershow/items/5bcefcbd6ae86a5ff202

file0.txt

pry(main)> Comment.all
=>
[#<
  id: 1,
  content: "あ">,
 #<
  id: 2,
  content: "い">,
 #<
  id: 3,
  content: "う">
]

file1.txt

Comment.order(id: "DESC")
Comment.order("id DESC")
Comment.all.order(id: "DESC")
Comment.all.order("id DESC")

file2.txt

Comment.order(id: "ASC")
Comment.order("id ASC")
Comment.all.order(id: "ASC")
Comment.all.order("id ASC")

file3.txt

Comment.all.reverse_order