title: Assign a key name in pre put hook: inheritance from a general ndb model
body: <p>I have this (working) model:</p>
  
  <pre><code>class WorkingModel(ndb.Model):
      # ...some properties...
      def _pre_put_hook(self):
          self.key = ndb.Key(WorkingModel, slugify(self.name))
  </code></pre>
  
  <p>What if I want to generalize the solution via inheritance? i.e.:</p>
  
  <pre><code>class slugModel(ndb.Model):
      def _pre_put_hook(self):
          self.key = ndb.Key(???, slugify(self.name))
  
  class WorkingModel(slugModel):
      name = ndb.StringProperty()
  </code></pre>