合并@ManyToOne方面的托管实体

级联应该始终从父级传播到子级，而不是相反。

在您的情况下，您需要从子级移除级联：

@JoinColumn(name = "department_id", referencedColumnName = "department_id")@ManyToOne(fetch = FetchType.LAZY)private Department department;

并确保设置了关联的两面：

Employee employee = entityManager.find(Employee.class, 1L);employee.setDepartment(department);employee.setEmployeeName("xyz");department.getEmployeeList().add(employee);entityManager.merge(department);