title: WithOptional with Entity Framework Core
body: <p>I'm trying to migrate my old app to the new EF Core but I cannot find some relationships like:</p>
  
  <pre><code>  HasRequired(o =&gt; o.Document).WithOptional(o =&gt; o.CancelNote);
  </code></pre>
  
  <p>Is there some extension methods? I cannot find on the docs.</p>
  
  <p>The <code>HasRequired</code> I think that is possible to substitute with <code>HasOne()</code> method, but how about the <code>WithOptional()</code>?</p>
  
  <p>Other thing, according to the <a href="http://ef.readthedocs.io/en/latest/modeling/relationships.html" rel="noreferrer">docs</a> the entity not uses the <code>virtual</code> keyword to create the navigation properties, how lazy load will work?</p>