I have a JPA-persisted object model that contains a many-to-one relationship: an Account has many Transactions. A Transaction has one
My Spring Data JPA-based answer: I simply added a @Transactional annotation to my outer method.
The child entity was immediately becoming detached because there was no active Hibernate Session context. Providing a Spring (Data JPA) transaction ensures a Hibernate Session is present.
https://vladmihalcea.com/a-beginners-guide-to-jpa-hibernate-entity-state-transitions/