find Composite Primary Keys

Quoting Learn by Doing <>:

I have a problem running find using composite primary keys. I have the
following models:

Code : - fold - unfold

   1. class Stat < ActiveRecord::Base
   2. set_primary_keys :user_id, :score_aspect
   3. belongs_to :user

Code : - fold - unfold

   1. class User < ActiveRecord::Base
   2. has_many :stats

Stat.find runs fine when it can find a stat. But it returns error
instead of "nil", when it cannot find a stat.

This is normal behavior, when calling find with a non-existant primary key, it
throws an exception, ActiveRecord::RecordNotFound. Try it on one of your
other classes, e.g., User.find(99999999).