How can I query/filter an exact count by an array of values in Elasticsearch?

Question

I have these documents:

{"title": "aa","cast": ["a","b","c","a"]}
{"title": "bb","cast": ["a","b","c","b"]}
{"title": "cc","cast": ["a","b","c","c"]}

I get the result using the faceting.

facets": {
      "cast_result": {
         "_type": "terms",
         "missing": 0,
         "total": 9,
         "other": 0,
         "terms": [
            {
               "term": "c",
               "count": 3
            },
            {
               "term": "b",
               "count": 3
            },
            {
               "term": "a",
               "count": 3
            }
         ]
      }
  }
}

but I want the result that the count of "a" is four,Because the frequency of "a" is four in the real document."b" and "c" also like the "a".

Answer 1

But I find a question ,when the number of the shards is more than 2, the answer is wrong ,so it must be lower than 2.It doesn't matter about the number of replicas.