code: <T extends Entity> EntityCollection<T> getMultiRelation(String name,
              boolean cachable) {
          return instance.getMultiRelation(this, name, cachable);
      }
docs: Get a multi-value relation by name for this entity. Ignore cache if
  cachable is false.
  
  @param name Name of the relation attribute.
  @param cachable False if should not be cached.
  @return IEntityCollection of T.