yii\mongodb\QueryBuilder::buildSelectFields PHP Method

    public function buildSelectFields($fields)
    {
        $selectFields = [];
        foreach ((array) $fields as $key => $value) {
            if (is_int($key)) {
                $selectFields[$value] = true;
            } else {
                $selectFields[$key] = is_scalar($value) ? (bool) $value : $value;
            }
        }
        return $selectFields;
    }