title: When should I use ndb.KeyProperty vs. ancestors?
body: <p>In AppEngine's NDB datastore, it seems there are 2 ways to relate objects with one another. There's <code>ndb.KeyProperty</code> and <a href="https://developers.google.com/appengine/docs/python/ndb/entities">parents/ancestors keys</a>. I'm a bit confused as to when I should be using one or the other? </p>
  
  <p>Right now I've been using <code>KeyProperty</code> exclusively, since it's the most familiar one, but I want to know when one is a better fit than the other.</p>