title: Symfony2 - extending entities, abstract entity
body: <p>I have 2 entities with the same fields - parent, children, order. Actually it is a mechanism and this 3 fields not applicable to content of this entity - like name, title, category etc.</p>
  
  <p>I want to set this fields to one place, one class and I'm considering where should I put it. Should it be an abstract class? Or should I make a trait?</p>
  
  <p>I also can use <strong>ORM\Discriminator</strong> mechanism, but I think this is for something else, not for that what I want to do.</p>