title: Get an Objectify Entity's Key
body: <p>Dummy question. </p>
  
  <p>I create my POJO Objectify entity (for example, "Category") and persist it.</p>
  
  <p>Then I retrieve it via a query.</p>
  
  <p>I want to use it in a one-to-may relationship e.g. want to set my category to one or more "Products".</p>
  
  <p>I will have this in my "Product"'s code: <code>Key&lt;Categoria&gt; categoria;</code></p>
  
  <p>So the question is: how can I find my retrieved entity's key for setting it in my product?</p>