title: how to make get() query with entity in ponyorm
body: <p>I'm using ponyorm. How can I convert this query into one that returns only one result if it exists? 
  I want it to throw an error if there is more than one result, similar to the <code>entity.get()</code> query. </p>
  
  <pre><code>url = 'some url'
  access= select(c for c in Access if c.people_url.lower().startswith(url))
  </code></pre>
  
  <p>Thank you. </p>