doctrine-hydrator.config.php

'doctrine-hydrator' => array(
        'product' => array(
            'entity_class' => 'doctrine.entity',
            'object_manager' => 'doctrine.entitymanager.orm_default',
            'by_value' => true,
            'hydrator' => 'Zend\\Stdlib\\Hydrator\\ArraySerializable',
            'strategies' => array(
                'category' => 'Application\Hydrator\EmbeddedEntityStrategy',
            )
        ),
        'product-category' => array(
            'entity_class' => 'doctrine.entity',
            'object_manager' => 'doctrine.entitymanager.orm_default',
            'by_value' => false,
            'hydrator' => 'Zend\\Stdlib\\Hydrator\\ArraySerializable',
            'strategies' => array(
            )
        ),

router.config.php

'router' => array(
        'routes' => array(
            'api.rest.product' => array(
                'type' => 'Segment',
                'options' => array(
                    'route' => '/api/product[/:product_id]',
                    'defaults' => array(
                        'controller' => 'Api\\V1\\Rest\\Product\\Controller',
                    ),
                ),
                'may_terminate' => true,
                'child_routes' => array(
                    'category' => array(
                        'type' => 'Segment',
                        'options' => array(
                            'route' => '/category[/:category_id]',
                        ),
                    ),
                )
            ),

zf-hal.config.php

'zf-hal' => array(
        'renderer' => array(
            'default_hydrator' => 'DoctrineModule\\Stdlib\\Hydrator\\DoctrineObject',
            'display_embedded_resources' => false,
            'render_embedded_resources' => false,
            'render_embedded_entities' => false,
            'render_collections' => true, // Collection Endpoints should be rendered
        ),
        'metadata_map' => array(
            'Application\\Entity\\Product' => array(
                'entity_identifier_name' => 'id',
                'route_name' => 'api.rest.product',
                'route_identifier_name' => 'product_id',
                'hydrator' => 'product'
            ),
            'Application\\Entity\\ProductCategory' => array(
                'entity_identifier_name' => 'id',
                'route_name' => 'api.rest.product/category',
                'route_identifier_name' => 'category_id',
                'hydrator' => 'product-category',
            ),