Hibernate @OneToMany reference child not updated / lost?

Say I have three entities.

@Entity
public class Process {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
@Column(unique = true)
private String name;
@ManyToAny(
metaColumn = @Column(name = "node_type"),
fetch = FetchType.LAZY
)
@AnyMetaDef(
idType = "long", metaType = "string",
metaValues = {
@MetaValue(targetEntity = Milestone.class, value = MILESTONE_DISC),
@MetaValue(targetEntity = Phase.class, value = PHASE_DISC)
}
)
@Cascade({org.hibernate.annotations.CascadeType.ALL})
@JoinTable(
name = "process_nodes",
joinColumns = @JoinColumn(name = "process_id", nullable = false),
inverseJoinColumns = @JoinColumn(name = "node_id", nullable = false)
)
private Collection<ProcessNode> nodes = new ArrayList<>();
...
}

@Entity
@ToString
@DiscriminatorValue(MILESTONE_DISC)
public class Milestone implements ProcessNode {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;
@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
private Collection<ResultDefinition> results;
@ManyToOne()
private Process process;
...
}

@Entity
@ToString
public class ResultDefinition {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String externalId;
private String name;
private ResultType resultType;
}

From my client I want to add an Object of type ResultDefinition to a Milestone in a Process like this:

@Transactional
@PostMapping("/{milestone_id}/results")
public ResultDefinitionDto createResult(@PathVariable("milestone_id") Long milestoneId, @RequestBody ResultDefinitionDto dto) {
Process foundProcess = getProcess(milestoneId);
checkFoundProcess(milestoneId, foundProcess);
Milestone milestone = getMilestone(foundProcess, milestoneId);
ResultDefinition resultDefinition = resultDefinitionMapper.fromDTO(dto);
milestone.addResult(resultDefinition);
processService.save(foundProcess);
//TODO: Find out why this is necessary (???)
ResultDefinition savedResult = milestone.getResult(resultDefinition.getName());
return resultDefinitionMapper.fromEntity(savedResult);
}

In my method createResult I add resultDefinition to the milestone results collection.
When I save the parent foundProcess, I see that foundprocess->milestone->resultDefinition get's persisted and gets an ID. When I call resultDefinition.getId() it returns null. Also the ResultDefinition Object in the foundProcess is another reference and not the same that I added to milestone.results.

Why do I get the correct instance when calling milestone.getResult()?

Edit: my implementation of processService / repository

@Override
public Process save(Process entity) {
return processRepository.saveAndFlush(entity);
}

public interface ProcessRepository extends JpaRepository<Process, Long>, JpaSpecificationExecutor<Process> {
...
}

The ResultDefinition gets replaced during the save process. The transient entity gets inserted into the database and will be replaced through a managed entity with an id. Your reference to the ResultDefinition in the createResult method still points to the transient one. That´s why you have to work with the returned entities from a save call.

In your case you are saving the parent process. So you have to access the saved ResultDefinition through the process or milestone entity.

You can try add flush method after save.