Although the discussion leading up to Listing 11.43 was couched in hypothetical terms, it actually works! You can verify this yourself by running the test suite, which should pass:

In some applications, this initial implementation might be good enough for most practical purposes. But it’s not the final implementation; see if you can make a guess about why not before moving on to the next section. (Hint: What if a user is following 5,000 other users?)