Database Exception – yii\db\Exception

     * Returns the PDO instance for the currently active master connection.
     * This method will open the master DB connection and then return [[pdo]].
     * @return PDO the PDO instance for the currently active master connection.
     */
    public function getMasterPdo()
    {
        $this->open();
        return $this->pdo;
    }
 
    /**
     * Returns the currently active slave connection.
     * If this method is called the first time, it will try to open a slave connection when [[enableSlaves]] is true.

     * is available and `$fallbackToMaster` is false.
     */
    public function getSlavePdo($fallbackToMaster = true)
    {
        $db = $this->getSlave(false);
        if ($db === null) {
            return $fallbackToMaster ? $this->getMasterPdo() : null;
        } else {
            return $db->pdo;
        }
    }
 
    /**

 
        if ($this->db->getTransaction()) {
            // master is in a transaction. use the same connection.
            $forRead = false;
        }
        if ($forRead || $forRead === null && $this->db->getSchema()->isReadQuery($sql)) {
            $pdo = $this->db->getSlavePdo();
        } else {
            $pdo = $this->db->getMasterPdo();
        }
 
        try {
            $this->pdoStatement = $pdo->prepare($sql);

                    Yii::trace('Query result served from cache', 'yii\db\Command::query');
                    return $result[0];
                }
            }
        }
 
        $this->prepare(true);
 
        $token = $rawSql;
        try {
            Yii::beginProfile($token, 'yii\db\Command::query');
 
            $this->pdoStatement->execute();

     * @return string|null|false the value of the first column in the first row of the query result.
     * False is returned if there is no value.
     * @throws Exception execution failed
     */
    public function queryScalar()
    {
        $result = $this->queryInternal('fetchColumn', 0);
        if (is_resource($result) && get_resource_type($result) === 'stream') {
            return stream_get_contents($result);
        } else {
            return $result;
        }
    }

 
        $this->select = $select;
        $this->limit = $limit;
        $this->offset = $offset;
 
        if (empty($this->groupBy) && empty($this->having) && empty($this->union) && !$this->distinct) {
            return $command->queryScalar();
        } else {
            return (new Query)->select([$selectExpression])
                ->from(['c' => $this])
                ->createCommand($command->db)
                ->queryScalar();
        }

    /**
     * @inheritdoc
     */
    protected function queryScalar($selectExpression, $db)
    {
        if ($this->sql === null) {
            return parent::queryScalar($selectExpression, $db);
        }
        /* @var $modelClass ActiveRecord */
        $modelClass = $this->modelClass;
        if ($db === null) {
            $db = $modelClass::getDb();
        }

     * @param Connection $db the database connection used to generate the SQL statement.
     * If this parameter is not given, the `db` application component will be used.
     * @return mixed the minimum of the specified column values.
     */
    public function min($q, $db = null)
    {
        return $this->queryScalar("MIN($q)", $db);
    }
 
    /**
     * Returns the maximum of the specified column values.
     * @param string $q the column name or expression.
     * Make sure you properly [quote](guide:db-dao#quoting-table-and-column-names) column names in the expression.

        $args = $this->controller->bindActionParams($this, $params);
        Yii::trace('Running action: ' . get_class($this->controller) . '::' . $this->actionMethod . '()', __METHOD__);
        if (Yii::$app->requestedParams === null) {
            Yii::$app->requestedParams = $args;
        }
 
        return call_user_func_array([$this->controller, $this->actionMethod], $args);
    }
}

        }
 
        $result = null;
 
        if ($runAction && $this->beforeAction($action)) {
            // run the action
            $result = $action->runWithParams($params);
 
            $result = $this->afterAction($action, $result);
 
            // call afterAction on modules
            foreach ($modules as $module) {
                /* @var $module Module */

        $parts = $this->createController($route);
        if (is_array($parts)) {
            /* @var $controller Controller */
            list($controller, $actionID) = $parts;
            $oldController = Yii::$app->controller;
            Yii::$app->controller = $controller;
            $result = $controller->runAction($actionID, $params);
            Yii::$app->controller = $oldController;
 
            return $result;
        } else {
            $id = $this->getUniqueId();
            throw new InvalidRouteException('Unable to resolve the request "' . ($id === '' ? $route : $id . '/' . $route) . '".');

            $params = $this->catchAll;
            unset($params[0]);
        }
        try {
            Yii::trace("Route requested: '$route'", __METHOD__);
            $this->requestedRoute = $route;
            $result = $this->runAction($route, $params);
            if ($result instanceof Response) {
                return $result;
            } else {
                $response = $this->getResponse();
                if ($result !== null) {
                    $response->data = $result;

        try {
 
            $this->state = self::STATE_BEFORE_REQUEST;
            $this->trigger(self::EVENT_BEFORE_REQUEST);
 
            $this->state = self::STATE_HANDLING_REQUEST;
            $response = $this->handleRequest($this->getRequest());
 
            $this->state = self::STATE_AFTER_REQUEST;
            $this->trigger(self::EVENT_AFTER_REQUEST);
 
            $this->state = self::STATE_SENDING_RESPONSE;
            $response->send();