讲解spring boot hibernate sql查询

Question

我正在使用 Spring Boot 实现基本的员工管理 CRUD API。 以下是我希望您关注的实体和服务。

我正在尝试创建这种双向关系：

An employee belongs to a single department, a department has many employees

员工/Employee.java

@Entity
@Table(name = "employee")
public class Employee {
    ...
    @ManyToOne
    @JoinColumn(name = "department_id")
    @JsonIgnoreProperties("employees")
    private Department department;
    ...
}

部门/部门.java

@Entity
@Table(name = "department")
public class Department {
    ...
    @OneToMany(mappedBy = "department", targetEntity = Employee.class, cascade = CascadeType.PERSIST)
    @JsonIgnoreProperties("department")
    private Set<Employee> employees;
    ...
}

部门/DepartmentService.java

...

public Employee addEmployeeToDepartment(UUID id, UUID empId) {
        Employee emp = employeeRepository.findById(empId).orElseThrow(EmployeeNotFoundException::new);
        Department dept = departmentRepository.findById(id).orElseThrow(DepartmentNotFoundException::new);
        emp.setDepartment(dept);
        return employeeRepository.save(emp);
}

...

我的问题：

当我尝试将员工添加到新部门时，会运行以下查询：

Hibernate: select e1_0.id,e1_0.address,e1_0.date_of_join,d1_0.id,d1_0.name,e1_0.email,e1_0.first_name,e1_0.last_name,e1_0.phone_number from employee e1_0 left join department d1_0 on d1_0.id=e1_0.department_id where e1_0.id=?
Hibernate: select d1_0.id,d1_0.name from department d1_0 where d1_0.id=?
Hibernate: update employee set address=?,date_of_join=?,department_id=?,email=?,first_name=?,last_name=?,phone_number=? where id=?

但是当我尝试将员工添加到该员工当前所在的同一部门时，会运行以下查询：

Hibernate: select e1_0.id,e1_0.address,e1_0.date_of_join,d1_0.id,d1_0.name,e1_0.email,e1_0.first_name,e1_0.last_name,e1_0.phone_number from employee e1_0 left join department d1_0 on d1_0.id=e1_0.department_id where e1_0.id=?

你能解释一下这是怎么发生的吗？ 如果需要的话，也许可以向我指出相关的休眠文档。

我预计每次都会运行 3 个查询：

fetch employee
fetch department
update employee

正如我在服务中所写的那样。但从日志来看似乎并非如此。

Answer 1

在您的

Employee

实体中，注释为

department

的

@ManyToOne

字段默认会立即获取。这就是为什么当您尝试获取

Employee

时会看到 join 子句。 在您的

Department

实体中，您有

@OneToMany

，默认情况下，其获取类型为惰性，因此没有连接，只有普通的旧选择位置，没有其他表中的任何其他内容。