gistfile1.txt

# works
POST /movies/movie/_search
{
  "query" : {
    "match" : {
      "inventory" : "MPA"
    }
  }
}

gistfile2.txt

# works
POST /movies/movie/_search
{
    "query": {
        "query_string": {
            "query": "MPA",            
            "fields": [
               "inventory"
            ]
        }
    }
}

gistfile3.txt

# doesn't work
POST /movies/movie/_search
{
    "query": {
        "query_string": {
            "query": "MPA"
        }
    }
}

gistfile4.txt

PUT /movies
{
   "settings": {
      "analysis": {
         "filter": {
            "ngram_filter": {
               "type": "ngram",
               "min_gram": 2,
               "max_gram": 20            }
         },
         "analyzer": {
            "index_ngram": {
               "type": "custom",
               "tokenizer": "keyword",
               "filter": [
                  "lowercase",
                  "ngram_filter"
               ]
            },
            "search_ngram": {
               "type": "custom",
               "tokenizer": "keyword",
               "filter": "lowercase"
            }
         }
      }
   },
   "mappings": {
      "movie": {
         "properties": {
            "inventory": {
               "type": "string",
               "analyzer": "index_ngram"
            }
         }
      }
   }
}

mapping.json

{
   "movies": {
      "mappings": {
         "movie": {
            "properties": {
               "director": {
                  "type": "string"
               },
               "genres": {
                  "type": "string"
               },
               "inventory": {
                  "type": "string",
                  "analyzer": "index_ngram"
               },
               "title": {
                  "type": "string"
               },
               "year": {
                  "type": "long"
               }
            }
         }
      }
   }
}