Spring-Data-Jpa bidirectional association with EmbeddedId. Foreign key is null when merging - spring

Question

I am running the latest version of Hibernate with 2 entities: Project and ProjectShare, which have a one-to-many relation set up bidirectional.
A ProjectShare is uniquely identified by a composite ID containing project_id and user_id. Besides the key, a ProjectShare contains a boolean flag whether the user gets read or write access on the project.
#Entity
#Table(name = "projects")
public class Project {
#Id
#GeneratedValue // UUID generator
#Column(name = "project_id")
private String id;
#Column(name = "name")
private String name;
#OneToMany(mappedBy = "project", cascade = CascadeType.ALL, orphanRemoval = true)
private List<ProjectShare> shares = new ArrayList<>();
public Project(String name) {
this.name = name;
}
public void addShare(ProjectShare share) {
shares.add(share);
share.setProject(this);
}
public void removeShare(ProjectShare share) {
shares.remove(share);
share.setProject(null);
}
}
#Entity
#Table(name = "project_shares")
public class ProjectShare {
#EmbeddedId
private ProjectShareId id;
#Column(name = "has_write_access")
private boolean hasWriteAccess;
#MapsId("projectId")
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = "project_id", nullable = false, updatable = false)
private Project project;
public ProjectShare(ProjectShareId id, boolean hasWriteAccess) {
this.id = id;
this.hasWriteAccess = hasWriteAccess;
}
public void setProject(Project project) {
this.project = project;
}
}
#Embeddable
public class ProjectShareId implements Serializable {
#Column(name = "project_id")
private String projectId;
#Column(name = "user_id")
private String userId;
public ProjectShareId(String userId) {
this.userId = userId;
}
// equals and hashCode go here...
}
If I create a new Project and assign new ProjectShare associations to it, everything works fine:
Project project = new Project("my_project");
project.addShare(new ProjectShare(new ProjectShareId("user1"), false));
project.addShare(new ProjectShare(new ProjectShareId("user2"), false));
projectRepository.save(project); // assume project id is '123'
Since all objects are new and not yet persisted, it executes 1 insert for the Project itself and 1 insert for each ProjectShare. Lets assume the project is inserted with id '123'.
Now, if I load this existing project, and add new ProjectShares to it, things go wrong:
Project project = projectRepository.findById("123");
project.addShare(new ProjectShare(new ProjectShareId("user2"), true));
project.addShare(new ProjectShare(new ProjectShareId("user3"), true));
projectRepository.save(project);
For every ProjectShare, this executes a SELECT on the values in ProjectShareId (project_id and user_id), followed by either an INSERT of UPDATE, depending on whether the record was found. This is the basic merge strategy of Hibernate and this is what we want.
The desired outcome of this should be:
Leave ProjectShare for user1 untouched
Update the ProjectShare for user2 (from false to true)
Create a new ProjectShare for user3
However, when the SELECT is executed for the ProjectShare, the foreign key project_id is always null. This means that existing records are never found, an INSERT is attempted instead of and UPDATE, and a DB-level Constraint violation is triggered.
How should I solve this issue? Should I manually go through the project.getShares() collection, find existing records and update them instead? I was hoping Hibernate would do this through its merge strategy.
Or could this be a bug in Hibernate related to associations with foreign keys in Embbedded IDs?