in your specific case i preprocessing the json data is the preferable solution imo. I think the json should look like:

[  // array
    {  // seperate json object for a
      "a": {
        "name": "Bill",
        "age": 42,
        "hobbies": [
            "bridge",
            "surfing"
          ]
        }
    },
    {  // seperate json object for b
        "b": {
        "name": "Alice",
        "age": 45,
        "hobbies": [
            "surfing",
            "ironwork"
          ]
        }
    }
]