This is a bit annoying - the code which associates the highlight assumes a `getId` method on the transformed object ([see here](https://github.com/FriendsOfSymfony/FOSElasticaBundle/blob/master/Doctrine/AbstractElasticaToModelTransformer.php#L105)), but `HighlightableModelInterface` doesn't specify that method. Suggest either adding it to the interface, or using the same property accessor approach with this class' identifier option to make it more flexible?