change password hashing on the fly gist

chage_password_devise.rb

     def valid_password?(incoming_password)
       result = super incoming_password
       if !result
         # try old encryptor during transition
         digest = Devise::Encryptors::CakeLegacy.digest(incoming_password, self.class.stretches, self.password_salt, self.class.pepper)
         result = Devise.secure_compare(digest, self.encrypted_password)
         if result
           # update password to use new encryptor when there is a match
           self.password = incoming_password
           self.save
         end
       end
       result
     end