title: How do you get an associated entity's id without loading the associated entity?
body: <p>Consider an entity with a (unidirectional) manyToOne association:</p>
  
  <pre><code>EntityX:
      type: entity
      table: x
      id:
          id:
              type: integer
              generator: { strategy: AUTO }
  
      fields:
          type:
              type: string
  
      manyToOne:
          entityY:
              targetEntity: EntityY
  </code></pre>
  
  <p>Any property of EntityY will be lazy loaded via a proxy class; however, EntityY's identifier will be (has to be) available before the proxy load. What is the best way to get at this identifier without triggering EntityY's proxy loader?</p>
  
  <p>Edit: I'm using Symfony2 (2.0.11) so the Doctrine dependency is constrained to v2.1</p>