title: entityManager.persist(user) -> javax.persistence.EntityExistsException: User@b3089 is already persistent
body: <pre><code>// lookup existing user &amp; set a currently null child entity (ContactInfo)
  user.setContactInfo(contactInfo);
  
  // update the user in the datastore with newly created child Entity             
  entityManager.persist(user);
  </code></pre>
  
  <p>Generates exception:</p>
  
  <pre><code>javax.persistence.EntityExistsException: User@b3089 is already persistent
  </code></pre>
  
  <p>Since the user already exists, is there some other method besides <code>entityManager.persist</code> that I should be using for an update instead of an insert?</p>