NullpointerException with PATCH and nested collections [DATAREST-1113]

Andrew Markwell opened DATAREST-1113 and commented

When performing a PATCH request with a 3-level nested graph of objects, a NullPointerException is encountered. Only the top-level JPA entity has a repository, and data the second 2 levels is inlined.

I've reproduced the bug with a small example on github: https://github.com/Spacemanbenny/sdr-patch-with-nested-collection @RepositoryRestResource(exported = false). run the PatchTest junit to reproduce. The junit doesn't fail, but the console output does show the NullPointerException.

Data structure is as follows:
[Company] --> [Employee] --> [Pen] ,
where each relation is OneToMany and unidirectional.

Steps to reproduce (in the Github example):
(a) Insert some starting data with Spring-Data JPA (not spring data rest)
1 company, 2 employees linked to companyy, 4 pens, 2 linked to first employee, 2 linked to second employee.

(b) Perform PATCH request containing:
2 employees:
First employee is not in the request (expect employee to be deleted and 2 pens to be deleted)
Second employee is unchanged
Third employee has been added with 1 new pen (expect 1 row added to employee and 1 row to pen)

What actually happens in this error:

Caused by: java.lang.NullPointerException: null
	at org.hibernate.collection.internal.PersistentBag.toArray(PersistentBag.java:284) ~[hibernate-core-5.0.12.Final.jar:5.0.12.Final]
	at java.util.ArrayList.<init>(ArrayList.java:177) ~[na:1.8.0_72]
	at org.springframework.data.rest.webmvc.json.DomainObjectReader.handleArrayNode(DomainObjectReader.java:324) ~[spring-data-rest-webmvc-2.6.5.RELEASE.jar:na]
	at org.springframework.data.rest.webmvc.json.DomainObjectReader.handleArray(DomainObjectReader.java:303) ~[spring-data-rest-webmvc-2.6.5.RELEASE.jar:na]
	at org.springframework.data.rest.webmvc.json.DomainObjectReader.doMerge(DomainObjectReader.java:244) ~[spring-data-rest-webmvc-2.6.5.RELEASE.jar:na]
	at org.springframework.data.rest.webmvc.json.DomainObjectReader.handleArrayNode(DomainObjectReader.java:345) ~[spring-data-rest-webmvc-2.6.5.RELEASE.jar:na]
	at org.springframework.data.rest.webmvc.json.DomainObjectReader.handleArray(DomainObjectReader.java:303) ~[spring-data-rest-webmvc-2.6.5.RELEASE.jar:na]
	at org.springframework.data.rest.webmvc.json.DomainObjectReader.doMerge(DomainObjectReader.java:244) ~[spring-data-rest-webmvc-2.6.5.RELEASE.jar:na]
	at org.springframework.data.rest.webmvc.json.DomainObjectReader.read(DomainObjectReader.java:87) ~[spring-data-rest-webmvc-2.6.5.RELEASE.jar:na]
	... 54 common frames omitted

Affects: 2.6.5 (Ingalls SR5)

Reference URL: https://github.com/Spacemanbenny/sdr-patch-with-nested-collection