TYPO3 Core

Stefan Bürk purposed a simple solution: Casting the CompositeExpression to string really functions!

$queryBuilder
  ->leftJoin(
        'tt_content',
        'tt_content',
        'tt_content_orig',
        (string)$queryBuilder->and(
          $queryBuilder->expr()->eq(
            'tt_content.t3_origuid',
            $queryBuilder->quoteIdentifier('tt_content_orig.uid')
          ),
          // ... further conditions
        ) 
  )

Works perfect. But I think it would be better to generally allow ConpositeExpressions for joinConditions and do the casting inside the join-methods of the queryBuilder.

Here another note of Stefan:

will look if this can be done - regarding doctrine/dbal changes. currently it would be possible, but TYPO3 QueryyBuilder does not extend doctrine querybuilder, with dbal v4 this is a must have. I will see.
Does not make sense to introduce it when it would block the upgrade for v13 then.

Mabye add the info as addition and let the issue stay open for now. Will assign myself later on it.

And here the promised code example:
I have a crowd of teachers in tx_myext_domain_model_teacher and a bunch of locations in tx_myext_domain_model_location. Each teacher can be available at several locations. So there is a MM-table tx_myext_domain_model_location_mm. This MM-table also contains relations for other tables / fields. Therefore tx_myext_domain_model_location_mm also contains the fiels tablenames and fieldname (defined as mm_match_fields in TCA of tx_myext_domain_model_teacher.

In my TCA I've defined, that if a location is selected, the teacher field appears via reload. But the options should only contain teachers that are available at the selected location. So I use a itemsProcFunc for this and it looks like ... Stefans (string)-casting already included ...

    public function getTeacherOptions(&$params): void
    {
        $locationUid = isset($params['row']['location']['0']) ? $params['row']['location']['0'] : $params['row']['location'];
        $connectionPool = GeneralUtility::makeInstance(ConnectionPool::class);
        $queryBuilder = $connectionPool->getQueryBuilderForTable('tx_myext_domain_model_teacher');
        $queryBuilder
            ->select('uid', 'surname', 'given_name', 'token')
            ->from('tx_myext_domain_model_teacher')
            ->orderBy('surname')
            ->addOrderBy('given_name');
        $constraints = [];
        if ($locationUid > 0) {
            $queryBuilder->leftJoin(
                'tx_myext_domain_model_teacher',
                'tx_myext_domain_model_location_mm',
                'mm',
                (string)$queryBuilder->expr()->and(
                    $queryBuilder->expr()->eq('mm.uid_foreign', $queryBuilder->quoteIdentifier('tx_myext_domain_model_teacher.uid')),
                    $queryBuilder->expr()->eq('mm.tablenames', $queryBuilder->createNamedParameter('tx_myext_domain_model_teacher', Connection::PARAM_STR)),
                    $queryBuilder->expr()->eq('mm.fieldname', $queryBuilder->createNamedParameter('locations', Connection::PARAM_STR))
                )
            );
            $constraints[] = $queryBuilder->expr()->eq('mm.uid_local', $queryBuilder->createNamedParameter($locationUid, \PDO::PARAM_INT));
        }

        // ...
        // further contraints
        // ...

        if (count($constraints)) {
            $queryBuilder->where(...$constraints);
        }
        $result = $queryBuilder->executeQuery();
        while ($row = $result->fetchAssociative()) {
            $params['items'][] = ['label' => $row['surname'] . ', ' . $row['given_name'] . ', ' . $row['token'], 'value' => $row['uid'], 'icon' => 'EXT:myext/Resources/Public/Icons/teacher.svg'];
        }
    }