如何在Spring JPA中保存引用现有实体的新实体？

Question

@ManyToMany(cascade = CascadeType.ALL)
@JoinTable(name = "employee_project", joinColumns = @JoinColumn(name = "Emp_Id"), inverseJoinColumns = @JoinColumn(name = "Proj_id"))
private Set<Project> projects = new HashSet<Project>();

@ManyToMany(mappedBy = "projects")
private Set<Employee> employees = new HashSet<Employee>();

detached entity passed to persist: Project

public void createNewEmployee(EmployeeDTO empDTO) {

  Employee emp = new Employee();
  // add stuff from DTO, including projects

  repository.saveAndFlush(emp);  // FAILS
}

public void updateEmployee(EmployeeDTO empDTO) {

   Employee emp = repository.findOne(empDTO.getId());
   // set stuff from DTO, including projects

   repository.saveAndFlush(emp);  // WORKS!
}

Answer 1

我想您在没有适当扩展事务边界的情况下与存储库进行交互。默认情况下，事务（因而会话）边界位于存储库方法级别。这会导致项目实例与EntityManager分离，因此不能将其包含在持久化操作中。

这里的解决方案是将事务边界扩展到客户端：

@Component
class YourRepositoryClient {

  private final ProjectRepository projects;
  private final EmployeeRepository employees;

  // … constructor for autowiring

  @Transactional
  public void doSomething() {
    Project project = projects.findOne(1L);
    Employee employee = employees.save(new Employee(project));
  }
}

这种方法使Project实例保持托管实体，从而为正确处理新的Employee实例执行持久化操作。