為什么外鍵數據將無法持久保存到mysql數據庫？

Question

我正在使用Java使用基本的mySQL關系數據庫。 我已為具有1：M關系的兩個表附加了實體類。 我已經在實體表中定義了“ 1對很多”關系。 在Projects類中，我想同時聲明外鍵（私有int contractor_id）以及getter和setter（如注釋所示），但是我不斷收到編譯錯誤，指出以下內容*項目[field.projects.com.cn]存在多個可寫映射。 contractor_id。 只能將其中一個定義為可寫，所有其他都必須指定為只讀。*因此，由於在BusinessAccount類中已經設置了它們的值，因此我將它們注釋掉。 現在，已使用我顯示的“ addProject（）”方法將數據持久保存到數據庫中。 但是，字段contractor_id（即外鍵）作為null傳遞給Projects表。 我有一個用於會話的contractor_id的值（名為sessionContractorId），但由於沒有此表的設置程序，因此無法將其傳遞給數據庫。 關於如何將數據庫外鍵的值持久化的任何建議將不勝感激。

@Entity
@Table(name = "business_accounts")
public class BusinessAccount {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int id;

@Column(name = "first_name")
private String firstName;

@Column(name = "surname")
private String surname;

@OneToMany(mappedBy = "businessAccount", fetch = FetchType.EAGER, cascade = { CascadeType.ALL })
private List<Projects> projects;



public int getId() {
    return id;
}

public void setId(int id) {
    this.id = id;
}

public String getFirstName() {
    return firstName;
}

public void setFirstName(String firstName) {
    this.firstName = firstName;
}

public String getSurname() {
    return surname;
}

public void setSurname(String surname) {
    this.surname = surname;
}

public List<Projects> getProjects()
{
    if (projects == null)
    {
        projects = new ArrayList<Projects>();
    }

    return projects;
}

public void setProjects(List<Projects> projects)
{
    this.projects = projects;
}

}



@Entity
@Table(name = "projects")
public class Projects {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int project_id;

@Column(name = "project_name")
private String projectName;

@Column(name = "project_description")
private String projectDescription;

//@Column(name = "contractor_id")
//private int contractorId;

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumns({ @JoinColumn(name = "contractor_id", referencedColumnName="id") })
private BusinessAccount businessAccount;


public BusinessAccount getBusinessAccount() {
    if (businessAccount == null) {
        businessAccount = new BusinessAccount();
    }
    return businessAccount;
}

public void setBusinessAccount(BusinessAccount businessAccount) {
    this.businessAccount = businessAccount;
}

public int getProject_id() {
    return project_id;
}

public void setProject_id(int project_id) {
    this.project_id = project_id;
}

public String getProjectName() {
    return projectName;
}

public void setProjectName(String projectName) {
    this.projectName = projectName;
}

public String getProjectDescription() {
    return projectDescription;
}

public void setProjectDescription(String projectDescription) {
    this.projectDescription = projectDescription;
}

//public int getContractorId() {
    //return contractorId;
//}

//public void setContractorId(int contractorId) {
    //this.contractorId = contractorId;
//}

}



@ManagedBean
@ViewScoped
public class ProjectBean implements Serializable {

private static final long serialVersionUID = -2107387060867715013L;
private static final String PERSISTENCE_UNIT_NAME = "NeedABuilderUnit";
private static EntityManagerFactory factory;

private Projects projects;


private List<BusinessAccount> businessAccount;

public ProjectBean() {
    factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
    EntityManager em = factory.createEntityManager();

    List<BusinessAccount> businessAccount = em.createQuery("from BusinessAccount a", BusinessAccount.class)
            .getResultList();
    em.close();
    setBusinessAccount(businessAccount);
}

@PostConstruct
public void init() {
    projects = new Projects();
}

public String addProject() {
    factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
    EntityManager em = factory.createEntityManager();   
        em.getTransaction().begin();

        String sessionEmail=Util.getEmail();
        Query myQuery = em.createQuery("SELECT u FROM BusinessAccount u WHERE u.email=:email");
        myQuery.setParameter("email", sessionEmail);
        List<BusinessAccount> accounts=myQuery.getResultList();
        int sessionContractorId=accounts.get(0).getId();
        em.persist(projects);
        em.getTransaction().commit();
        em.close();

        return "success";
    }

Answer 1

我注意到有關代碼的兩件事。

首先，代碼應使用實體，並忽略特定實體的id字段。 因此，當您提取帳戶時，它應該獲取實體而不是ID：

//Don't do this
List<BusinessAccount> accounts=myQuery.getResultList();
int sessionContractorId=accounts.get(0).getId();

//Instead do this
List<BusinessAccount> accounts=myQuery.getResultList();
BusinessAccount account =accounts.get(0); //hopefully an account exists

其次，在JPA中，您負責管理關聯雙方。 因此，您必須將Account添加到Project並在關系的另一側為Account設置Project 。 我從沒在代碼中看到這種情況，僅看到持久化的projects （不確定其來源）。 假設projects是一個List<Project> ，它將看起來像這樣：

public String addProject() {
        factory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);
        EntityManager em = factory.createEntityManager();   
        em.getTransaction().begin();

        String sessionEmail=Util.getEmail();
        Query myQuery = em.createQuery("SELECT u FROM BusinessAccount u WHERE u.email=:email");
        myQuery.setParameter("email", sessionEmail);
        List<BusinessAccount> accounts=myQuery.getResultList();
        BusinessAccount account =accounts.get(0);

        projects.setBusinessAccount(account); //managing both sides
        account.getProjects().add(projects); //managing both sides
        em.persist(projects);
        em.getTransaction().commit();
        em.close();

        return "success";
    }

附帶說明一下，您可能希望將類名更改為Project並使用變量名project因為它可以更准確地描述關系。 同樣，由於您正在創建一個新Project您將需要實例化List<BusinessAccount> ：

List<BusinessAccount> projects = new ArrayList<BusinessAccount>();

希望這能解決您的問題，我建議您觀看我在雙向一對多關系上創建的此視頻教程 。

Answer 2

您應該使用@JoinColumn(name="id_user", insertable = false, updatable = false)

請參考： JPA中的多個可寫映射？

希望這可以幫助 ：）