Eager loading joined relations

[xdrew]

Hello!

Could you please give a small hint on how the projection should be extended to get joined relations?
For instance:

"hydra:member": [
    {
      "@id": "/api/restaurants/1",
      "@type": "Restaurant",
      "id": 1,
      "deliveryTime": 10,
      "config": {
        "@id": "/api/configs/1",
        "@type": "Config",
        "dynamicWaitingTime": 0
      },
  }
]

By I could only get field config_id instead.
I thought it should be done with following steps:

1. Add custom query with joins like docs says: https://github.com/pomm-project/ModelManager/blob/master/documentation/model_manager.rst#complex-queries
2. Hook into normalizer to convert plain joined fields into multidimensional array

But I have no idea how to achieve step 1 with api platform.
Thanks in advance.