title: Check if JPA entity exists without loading it
body: <p>The problem is following.</p>
  
  <p>We have a server endpoint, that receives feedback from clients. Feedback received in multipart/form-based format, with fields:</p>
  
  <pre><code>ProductId - product identifier
  Message - feedback message
  Log_file - attached log file
  Screenshot - attached screenshot file
  </code></pre>
  
  <p>Server code first checks if the product with given id exists and if not - closes connection without receiving any attached files.</p>
  
  <p>We use Eclipselink JPA to store product objects. </p>
  
  <p>How it's possible to check if a product with given id exists without loading it from underlying database?</p>