如何制作复合主键(java持久性注释)

Question

如何使表user_roles将两列(userID,roleID)定义为复合主键.应该很容易,只是记不住/找不到.

在user实体中:

@ManyToMany(fetch = FetchType.LAZY)
@JoinTable(name = "user_roles")
public List<RoleDAO> getRoles() {
    return roles;
}

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
public Integer getUserID() {
    return userID;
}

在roles实体中:

@ManyToMany(fetch = FetchType.LAZY)
@JoinTable(name = "user_roles")
public List<UserDAO> getUsers() {
    return users;
}

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
public Integer getRoleID() {
    return roleID;
}

谢谢.

**更多信息

因此,有一个第三表user_roles即花费(由上述自动生成)userID从user实体和roleID从roles实体.现在我需要生成的table(user_roles)中的这两列是一个复合主键.

Answer 1

你已经在这里找到了一些关于如何完全按照你的要求做出的好答案.

作为参考,我只想提一下在Hibernate中这样做的推荐方法,即使用代理键作为主键,并将业务键标记为NaturalId:

虽然我们建议使用代理键作为主键,但您应该尝试识别所有实体的自然键.自然键是属性或属性的组合,它们是唯一且非空的.它也是不可改变的.映射元素内的自然键的属性.Hibernate将生成必要的唯一键和可空性约束,因此,您的映射将更加自我记录.

建议您实现equals()和hashCode()以比较实体的自然键属性.

在代码中,使用注释,这看起来像这样:

@Entity
public class UserRole {
  @Id
  @GeneratedValue
  private long id;

  @NaturalId
  private User user;
  @NaturalId
  private Role role;
}

使用它可以为您节省很多麻烦,因为您会发现您经常需要引用/映射组合主键.

我发现这很困难,最后只是放弃了与Hibernate的斗争,而是决定顺其自然.我完全理解,在您的情况下这可能是不可能的,因为您可能正在处理遗留软件或依赖项,但我只是想提及它以供将来参考.(如果你不能使用它也许其他人可以!)

Answer 2

为了满足您的要求,您可以将@ManyToMany映射为@OneToMany映射.这样,USER_ROLE将包含USER_ID和ROLE_ID作为复合主键

我会告诉你如何:

@Entity
@Table(name="USER")
public class User {

    @Id
    @GeneratedValue
    private Integer id;

    @OneToMany(cascade=CascadeType.ALL, mappedBy="joinedUserRoleId.user")
    private List<JoinedUserRole> joinedUserRoleList = new ArrayList<JoinedUserRole>();

    // no-arg required constructor
    public User() {}

    public User(Integer id) {
        this.id = id;
    }

    // addRole sets up bidirectional relationship
    public void addRole(Role role) {
        // Notice a JoinedUserRole object
        JoinedUserRole joinedUserRole = new JoinedUserRole(new JoinedUserRole.JoinedUserRoleId(this, role));

        joinedUserRole.setUser(this);
        joinedUserRole.setRole(role);

        joinedUserRoleList.add(joinedUserRole);
    }

}

@Entity
@Table(name="USER_ROLE")
public class JoinedUserRole {

    public JoinedUserRole() {}

    public JoinedUserRole(JoinedUserRoleId joinedUserRoleId) {
        this.joinedUserRoleId = joinedUserRoleId;
    }

    @ManyToOne
    @JoinColumn(name="USER_ID", insertable=false, updatable=false)
    private User user;

    @ManyToOne
    @JoinColumn(name="ROLE_ID", insertable=false, updatable=false)
    private Role role;

    @EmbeddedId
    // Implemented as static class - see bellow
    private JoinedUserRoleId joinedUserRoleId;

    // required because JoinedUserRole contains composite id
    @Embeddable
    public static class JoinedUserRoleId implements Serializable {

        @ManyToOne
        @JoinColumn(name="USER_ID")
        private User user;

        @ManyToOne
        @JoinColumn(name="ROLE_ID")
        private Role role;

        // required no arg constructor
        public JoinedUserRoleId() {}

        public JoinedUserRoleId(User user, Role role) {
            this.user = user;
            this.role = role;
        }

        public JoinedUserRoleId(Integer userId, Integer roleId) {
            this(new User(userId), new Role(roleId));
        }

        @Override
        public boolean equals(Object instance) {
            if (instance == null)
                return false;

            if (!(instance instanceof JoinedUserRoleId))
                return false;

            final JoinedUserRoleId other = (JoinedUserRoleId) instance;
            if (!(user.getId().equals(other.getUser().getId())))
                return false;

            if (!(role.getId().equals(other.getRole().getId())))
                return false;

            return true;
        }

        @Override
        public int hashCode() {
            int hash = 7;
            hash = 47 * hash + (this.user != null ? this.user.hashCode() : 0);
            hash = 47 * hash + (this.role != null ? this.role.hashCode() : 0);
            return hash;
        }

    }

}

记得

如果对象具有指定的标识符或复合键,则在调用save()之前,应该将标识符分配给对象实例.

所以我们创建了一个像这样的JoinedUserRoleId构造函数来处理它

public JoinedUserRoleId(User user, Role role) {
    this.user = user;
    this.role = role;
}

最后是Role类

@Entity
@Table(name="ROLE")
public class Role {

    @Id
    @GeneratedValue
    private Integer id;

    @OneToMany(cascade=CascadeType.ALL, mappedBy="JoinedUserRoleId.role")
    private List<JoinedUserRole> joinedUserRoleList = new ArrayList<JoinedUserRole>();

    // no-arg required constructor
    public Role() {}

    public Role(Integer id) {
        this.id = id;
    }

    // addUser sets up bidirectional relationship
    public void addUser(User user) {
        // Notice a JoinedUserRole object
        JoinedUserRole joinedUserRole = new JoinedUserRole(new JoinedUserRole.JoinedUserRoleId(user, this));

        joinedUserRole.setUser(user);
        joinedUserRole.setRole(this);

        joinedUserRoleList.add(joinedUserRole);
    }

}

根据测试它,让我们写下面的内容

User user = new User();
Role role = new Role();

// code in order to save a User and a Role
Session session = HibernateUtil.getSessionFactory().openSession();
session.beginTransaction();

Serializable userId  = session.save(user);
Serializable roleId = session.save(role);

session.getTransaction().commit();
session.clear();
session.close();

// code in order to set up bidirectional relationship
Session anotherSession = HibernateUtil.getSessionFactory().openSession();
anotherSession.beginTransaction();

User savedUser = (User) anotherSession.load(User.class, userId);
Role savedRole = (Role) anotherSession.load(Role.class, roleId);

// Automatic dirty checking
// It will set up bidirectional relationship
savedUser.addRole(savedRole);

anotherSession.getTransaction().commit();
anotherSession.clear();
anotherSession.close();

请注意根据以上代码无参考JoinedUserRole类.

如果要检索JoinedUserRole,请尝试以下操作

Session session = HibernateUtil.getSessionFactory().openSession();
session.beginTransaction();

Integer userId;
Integer roleId;

// Lets say you have set up both userId and roleId
JoinedUserRole joinedUserRole = (JoinedUserRole) session.get(JoinedUserRole.class, new JoinedUserRole.JoinedUserRoleId(userId, roleId));

// some operations

session.getTransaction().commit();
session.clear();
session.close();

问候,