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 <virtualType name="Magento\Sales\Model\ResourceModel\Order\Grid" type="Magento\Sales\Model\ResourceModel\Grid">
    <arguments>
        <argument name="joins" xsi:type="array">
            <item name="table_that_you_want_to_join" xsi:type="array">
                <item name="table" xsi:type="string">table_that_you_want_to_join</item>
                <item name="origin_column" xsi:type="string">entity_id</item>
                <item name="target_column" xsi:type="string">parent_id</item>
            </item>
        </argument>
        <argument name="columns" xsi:type="array">

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public static function populateMappings(AutoloaderInterface $autoloader, DirectoryList $dirList)
    {
        $modulesDir = $dirList->getPath(DirectoryList::MODULES);
        $generationDir = $dirList->getPath(DirectoryList::GENERATION);
        $frameworkDir = $dirList->getPath(DirectoryList::LIB_INTERNAL);

        $autoloader->addPsr4('Magento\\', [$modulesDir . '/Magento/', $generationDir . '/Magento/'], true);

        $autoloader->addPsr0('Apache_', $frameworkDir, true);
        $autoloader->addPsr0('Cm_', $frameworkDir, true);

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Populator::populateMappings($autoloadWrapper, $dirList);

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$this->directories[self::SYS_TMP] = [self::PATH => realpath(sys_get_temp_dir())];

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 public static function getDefaultConfig()
    {
        $result = [
            self::ROOT => [parent::PATH => ''],
            self::APP => [parent::PATH => 'app'],
            self::MODULES => [parent::PATH => 'app/code'],
            self::CONFIG => [parent::PATH => 'app/etc'],
            self::LIB_INTERNAL => [parent::PATH => 'lib/internal'],
            self::VAR_DIR => [parent::PATH => 'var'],
            self::CACHE => [parent::PATH => 'var/cache'],

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return new DirectoryList($rootDir, $customDirs);

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public static function populateAutoloader($rootDir, $initParams)
{
    $dirList = self::createFilesystemDirectoryList($rootDir, $initParams);
    $autoloadWrapper = AutoloaderRegistry::getAutoloader();
    Populator::populateMappings($autoloadWrapper, $dirList);
}

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\Magento\Framework\App\Bootstrap::populateAutoloader(BP, []);

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AutoloaderRegistry::registerAutoloader(new ClassLoaderWrapper($composerAutoloader));

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public function register($prepend = false)
{
  spl_autoload_register(array($this, 'loadClass'), true, $prepend);
}