Question

I have the following code in my user model:

before_save :create_remember_token

  private
    def create_remember_token
      generate_token(:remember_token)
    end

    def generate_token(column)
      begin
        self[column] = SecureRandom.urlsafe_base64
      end while User.exists?(column => self[column])
    end

This works well, but the trouble is that I was expecting it to fail. I was under the impression that the line: while User.exists?(column=>self[column]) would return false, since the record has not yet been saved to the db, and thus, the remember_token would not be set.

But newly created users do have their remember_token set, and I’m a little bit confused as to how this can be possible. Am I missing something?

Answer 1

In case anyone else is wondering, the code inside a begin end expression will run once before the condition is evaluated, unless you provide a rescue clause:

# x is not incremented as condition is evaluated first

1.9.3p194 :033 > x=0
 => 0
1.9.3p194 :034 > x+=1 while x>14
 => nil 
1.9.3p194 :035 > x
 => 0 

# x is incremented because code inside begin end is evaluated once before the condition 
1.9.3p194 :036 > begin
1.9.3p194 :037 >     x+=1
1.9.3p194 :038?>   end while x>14
 => nil 
1.9.3p194 :039 > x
 => 1