Reported in https://github.com/mongodb/laravel-mongodb/issues/3665#issuecomment-6818120336

A PHPUnit suite run in parallel (~3,500 tests) reports:

The execution time is covered by PHPLARA-326. This ticket covers the connection growth.

Diagnosis

• The Queryable Encryption field type cache (#3645) does not open SQL connections. EncryptedFieldTypes::fieldTypesFor() only resolves the model connection, which DatabaseManager caches.
• Connection::syncCommandSubscriber() (#3646) only touches the MongoDB client.
• Prime suspect: #3627 (PHPLARA-148), the hybrid hasOneThrough() and hasManyThrough() relations added in 5.12.0.

Next steps

• Reproduce with an application that uses a through relation between a SQL model and a MongoDB model.
• Compare the connection count on 5.11.0 and 5.12.0.
• Disable the through relation on 5.12.0 and compare again.
• Check whether a SQL connection is opened for each relation call and not released.