Spring JPA 存储库多对多映射在添加记录时创建重复项答案

【问题标题】：Spring JPA repository many to many mapping creating duplicate while adding recordsSpring JPA 存储库多对多映射在添加记录时创建重复项
【发布时间】：2021-07-31 08:28:54
【问题描述】：

我有两个通过多对多关系关联的实体。现在我创建了一条新记录，我需要更新关系，但是在这样做的同时，我在一个子表中获得了每个父记录的重复条目。下面是实体代码

@Entity
@Table(name = "Employee")
public class Employee {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    private String title;
    @ManyToMany(fetch = FetchType.EAGER,
            cascade = {
                CascadeType.PERSIST,
                CascadeType.MERGE
            },
            mappedBy = "employees")
    private Set<Department> departments = new HashSet<>();

    public Employee() {
    }

    
    public Employee(String title) {
        this.title= title;
    }

    public Integer getId() {
        return id;
    }


    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }


    public Set<Department> getDepartments() {
        return departments;
    }


    public void setDepartments(Set<Department> departments) {
        this.departments = departments;
    }

    @Override
    public String toString() {
        return "Colmn [id=" + id + ", title=" + title + "]";
    }

    
}


@Entity
@Table(name = "Department")
public class Department {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;
    private String company;
    
    @ManyToMany(fetch = FetchType.EAGER,
            cascade = {
                CascadeType.PERSIST,
                CascadeType.MERGE
            })
    @Fetch(FetchMode.JOIN)
    @JoinTable(name = "dept_emp",
            joinColumns = { @JoinColumn(name = "dept_id") },
            inverseJoinColumns = { @JoinColumn(name = "emp_id") })
    private Set<Employee> employees = new HashSet<>();
    
    

    public Department() {
    }

    public Department(String company) {
        this.company = company;     
    }

    
    public long getId() {
        return id;
    }

    public String getCompany() {
        return company;
    }

    public void setCompany(long company) {
        this.company = company;
    }

    
    public Set<Employee> getEmployees() {
        return employees;
    }

    public void setEmployees(Set<Employee> employees) {
        this.employees = employees;
    }


    @Override
    public String toString() {
        return "Department [id=" + id + ", company=" + company + ", employees=" + employees + "]";
    }
}

我正在使用 spring curd 存储库

public interface DepartmentRepository extends CrudRepository<Department, Long> {
}


public interface EmployeeRepository extends CrudRepository<Employee, Long> {
}

现在我在数据库中有现有记录我想在系统中添加一个属于所有部门的新员工，所以我在下面做了

/**Search all the existing departments*/
    List<Department> depts = (List<Department>) departmentRepository.findAll();
    
    /**Create the new employee record*/
    Employee employee = new Employee();
    employee.setTitle("Adam");
    
    /**Iterate and create the relatonship*/
    for (Department d : depts) {
        d.getEmployees().add(employee);
        employee.getDepartments().add(d);
        departmentRepository.save(d);
    }

问题在于我在员工表中插入了一条记录的每个部门，即如果我有 5 个部门，那么我在员工表中插入了 5 次员工记录，即“Adam”。部门表没有任何问题。

如何处理

【问题讨论】：

在最后一行中，您正在保存一个名为“r”的对象......这个“r”来自哪里？
Srry 拼写错误，已更正，但问题仍然存在

标签： spring hibernate spring-data-jpa many-to-many

【解决方案1】：

自然会发生这种情况，您需要先保留新员工，然后再添加并保存部门。否则 jpa 对该员工一无所知，因此无法对其进行管理。

所以请调用employee = employeeRepository.save(employee)，然后将其添加到您想要的所有部门。

确保您添加的是从保存操作返回的实体 - 保存方法将返回持久化的实体

【讨论】：