Solr block-join

gistfile1.json

[
  {
    "id":"1",
    "feed":"parent",
    "childDocuments":[
      {
        "id":"1-1",
        "feed":"child"
      },
      {
        "id":"1-2",
        "feed":"child"
      }
    ]
  },
  {
    "id":"2",
    "feed":"parent",
    "childDocuments":[
      {
        "id":"2-1",
        "feed":"child",
        "childDocuments":[
          {
            "id":"2-1-1",
            "feed":"grandchild"
          },
          {
            "id":"2-1-1",
            "feed":"grandchild2"
          },
          {
            "id":"2-1-1",
            "feed":"grandchild3"
          }
        ]
      }
    ]
  }
]