"IdentifierGenerationException: attempted to assign id from null one-to-one property" in Hibernate

Issue

• Using @JoinColumn to explicitly set the column name for an inherited @Id property for a derived entity using @MapsId

  @Entity
  public class Employee {
          @Id
          public String getName() { ... }
          private void setName(String name) { ... }
  
          @OneToOne(mappedBy = "employee", cascade = CascadeType.ALL ...)
          public Task getTask() { ... }
          public void setTask(Task task) { ... }
          ...
  }
  
  @MappedSuperclass
  public abstract class BaseEntity {
          ...
          @Id
          @Column(name = "id", insertable = true, updatable = false)
          public String getId() { ... }
          private void setId(String id) { ... }
          ...
  }
  
  @Entity
  public class Task extends BaseEntity {
          ...
          @OneToOne
          @JoinColumn(name = "id", nullable = false) // otherwise key column would be employee_name
          @MapsId
          public Employee getEmployee() { ... }
          protected void setEmployee(Employee employee) { ... }
          ...
  }
  

• On merging an Employee instance after supplying an associated Task, encountering an exception

  ...  org.hibernate.id.IdentifierGenerationException: attempted to assign id from null one-to-one property ...