How can I sort a list of ActiveRecords by an attribute length?

I have an object like this:

irb(main):076:0> hints = Hint.where("sentence LIKE ?","%你%")
  Hint Load (4.0ms)  SELECT "hints".* FROM "hints" WHERE (sentence LIKE '%你%')
[
    [0] #<Hint:0x007fb99c6520b8> {
                :id => 214,
          :sentence => "我为你们立下模范，我向你们怎样做，你们也该照样做。",
              :user => nil,
           :learned => nil,
        :created_at => Sun, 06 Jan 2013 18:14:33 UTC +00:00,
        :updated_at => Sun, 06 Jan 2013 18:14:33 UTC +00:00
    },
    [1] #<Hint:0x007fb99c659a70> {
                :id => 229,
          :sentence => "他快要完成地上的传道工作时，曾向耶和华祷告说“我已经使他们使徒认识你的名，以后还要使他们认识”。",
              :user => nil,
           :learned => nil,
        :created_at => Sun, 06 Jan 2013 18:43:23 UTC +00:00,
        :updated_at => Sun, 06 Jan 2013 18:43:23 UTC +00:00
    },
    [2] #<Hint:0x007fb99c659458> {
                :id => 234,
          :sentence => "你的王到你这里来了。",
              :user => nil,
           :learned => nil,
        :created_at => Sun, 06 Jan 2013 18:48:12 UTC +00:00,
        :updated_at => Sun, 06 Jan 2013 18:48:12 UTC +00:00
    }
]
irb(main):077:0> hints.class
ActiveRecord::Relation < Object

How can I sort by sentence length?

My final goal is to make it so that when someone clicks on a Chinese character in a lesson that they will be shown a few of the shortest example sentences available as a hint to the meaning of the character.

I am using PostgreSQL.

this sort:

hints = hints.sort { |x,y| x.sentence.length <=> y.sentence.length }

will do the job