OneToOne shared primary key relationship breaks with field annotations

Using @OneToOne with @Id to share a primary key only seems to work if the getters (for the ID and relationship fields) are annotated. If the annotations are on the fields, then it fails. I don't know if this is accepted behaviour or not.

Based on this example, I put together a MWE as a github repo. The master branch uses getter annotations, the `field-annotations` branch uses field annotations. Run the app with `mvn spring-boot:run` to see it fail.

As far as I can tell, it is during the cascade merge that the link back to the parent (Book) is broken.