Single nested child can cause ClassCastException when requesting inner_hits

[tstibbs]

Just been trying out a 1.5 snapshot (built from commit 75b6d8e from the 1.x branch) to request inner hits.

If your mapping has a 'nested' type and you specify your child document as an object rather than an array, you get a ClassCastException. I.e. your doc looks like this:

{
  "nested_field": {
    "make": "ford"
  }
}

When searching you will get an exception like this:

java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to java.util.List
at org.elasticsearch.search.fetch.FetchPhase.createNestedSearchHit(FetchPhase.java:294)
at org.elasticsearch.search.fetch.FetchPhase.execute(FetchPhase.java:178)
at org.elasticsearch.search.fetch.innerhits.InnerHitsFetchSubPhase.hitExecute(InnerHitsFetchSubPhase.java:96)
at org.elasticsearch.search.fetch.FetchPhase.execute(FetchPhase.java:190)
at org.elasticsearch.search.SearchService.executeFetchPhase(SearchService.java:501)

Note that it works ok if you index your document like this (note the extra square brackets to indicate an array)

{
  "nested_field": [
    {
      "make": "ford"
    }
  ]
}

I think that either the nested hits stuff should support this single child using the non-array format, or elasticsearch should throw an error when you try to index a document using that syntax.

(note this issue came out of a comment on pull request #8153)