Spring Data Rest: JOIN search by parent column is throwing error (o.s.d.r.w.RepositoryRestExceptionHandler : PersistentEntity must not be null!) - spring-data-rest

Question

I am using Spring Data Rest for a Department-Employee relationship as shown below, and have a RepositoryRestResource defined on Employee.
class Department {
#Id
UUID Id;
String name;
#OneToMany(mappedBy = "employee", targetEntity = Employee.class, cascade = {CascadeType.ALL}, fetch = FetchType.LAZY)
#RestResource(exported = false)
private Set<Employee> employees;
}
class Employee {
#Id
UUID Id;
String firstName;
String lastName;
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = "dept_ref")
#RestResource(exported = false)
private Department department;
}
#Projection(name = "employeeProjection", types = {Employee.class})
interface EmployeeProjection {
String getFirstName();
String getLastName();
}
#RepositoryRestResource(excerptProjection = EmployeeProjection.class)
public interface EmployeeRepository extends CrudRepository<Employee, UUID> {
//this doesn't work
#Query(value = "SELECT emp.firstName as firstName , emp.lastName as lastName FROM Employee emp JOIN emp.department "
+ "WHERE emp.department.name = :departmentName ")
List<EmployeeProjection> findByDepartmentName(#Param("departmentName") String departmentName);
//this works
List<EmployeeProjection> findByFirstName(#Param("firstName") String firstName);
}
Issue is when I perform a join search as findByDepartmentName, I am getting below error.
2019-07-01 00:26:33.300 ERROR 7396 --- [nio-8443-exec-5] o.s.d.r.w.RepositoryRestExceptionHandler : PersistentEntity must not be null!
java.lang.IllegalArgumentException: PersistentEntity must not be null!
at org.springframework.util.Assert.notNull(Assert.java:134)
at org.springframework.data.rest.webmvc.PersistentEntityResource$Builder.<init>(PersistentEntityResource.java:140)

Answer 1

the issue got resolved by changing the return type to List<Employee> from List<EmployeeProjection>, also it still returns me the projection as its an excerptProjection. not sure why the Spring Data Rest expects this but it works.
List<Employee> findByDepartmentName(#Param("departmentName") String departmentName);