如何在不加载关联实体的情况下获取关联实体的id？

考虑具有（单向）manyToOne关联的实体：

Consider an entity with a (unidirectional) manyToOne association:

EntityX:
    type: entity
    table: x
    id:
        id:
            type: integer
            generator: { strategy: AUTO }

    fields:
        type:
            type: string

    manyToOne:
        entityY:
            targetEntity: EntityY

EntityY的任何属性将通过代理类进行延迟加载;然而，在代理加载之前，EntityY的标识符将（必须）可用。在触发EntityY的代理装载器的情况下，获取此标识符的最佳方式是什么？

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?

编辑：我正在使用Symfony2（2.0.11），所以Doctrine依赖被限制v2.1

I'm using Symfony2 (2.0.11) so the Doctrine dependency is constrained to v2.1