How to group and count results in Elasticsearch

SQL query

SELECT Field, COUNT(Field)
FROM Table
GROUP BY Field

Query in Elasticsearch

GET /books/_search
{
  "size": 0,
  "aggs": {
    "genre": {
      "terms": {
        "field": "genre"
      }
    }
  }
}

Result

{
  "error": {
    "root cause" [
     {
      "type": "illegal _argument_exception",
      "reason" : "Text fields are not optimised for operations that require per-document field data like aggregations and sorting, so these operations are disabled by default. Please use a keyword field instead. Alternatively, set fielddata=true on [title] in order to load field data by uninverting the inverted index. Note that this can use significant memory."
     },
    ],
    ...
  }
}

PUT /books
{
  "mappings": {
    "properties": {
      "author":    { "type": "text" },
      "genre":  { "type": "keyword"  },
      "title":   { "type": "text"  }
    }
  }
}