elasticsearch aggregation by first letters of search fields

gistfile1.txt

$field - the name of field to aggregate
$firstChars - the first letters to search aggregations values by

$data = [
    'query' => [
        'bool' => [
            'must' => [
                'query_string' => [
                    'query' => $field . ':' . $firstChars . '*'
                ]
            ]
        ]
    ],
    'size' => 0,
    'aggregations' => [
        $field => [
            'terms' => [
                'field' => $field,
                'size' => $limit,
                'order' => [
                    '_term' => 'asc'
                ]
            ]
        ]
    ]
];