public function getDefaultConnection()
    {
        if (null === $this->defaultConnectionName) {
            throw new \RuntimeException('There is not default connection name.');
        }

        if (!isset($this->connections[$this->defaultConnectionName])) {
            throw new \RuntimeException(sprintf('The default connection "%s" does not exists.', $this->defaultConnectionName));
        }

        return $this->connections[$this->defaultConnectionName];
    }