java - 仅从 @ManyToOne 关系中获取一列

Question

我正在尝试缩小我的实体之一，因为它不需要加载整个相关对象。我想要实现的不是加载映射为 @ManyToOne 关系的整个实体，而是仅加载其字段之一。假设我们有以下两个实体(来自样本的实体是组成的):

@Entity
public class Session {
    @Id
    private Long id;
    private String someField;
    @ManyToOne
    @JoinColumn(name = "sessionId", referencedColumnName = "id")
    private User user;
}

@Entity
public class User {
    @Id
    private Long id;
    private String username;
    private String address;
    private LocalDateTime lastLogin;
}

我希望最终得到较小的 session 实体，如下所示:

@Entity
public class Session {
    @Id
    private Long id;
    private String someField;
    @ManyToOne
    @JoinColumn(name = "sessionId", referencedColumnName = "id")
    private LocalDateTime lastLogin; // lastLogin value from User Entity
}

我试图通过混合@Column注释来实现这一点，但忘记了它在@ManyToOne上是不允许的。我正在考虑可以使用 @ElementCollection 完成类似的事情:

    @ElementCollection(fetch = FetchType.EAGER)
    @CollectionTable(name = "OTHER_TABLE")
    @Column(name = "someFieldFromOtherTable")
    private Set<String> someFieldFromOtherTableValues;

是否有可能从反射实体中仅获取一列？

Answer 1

这就是我的代码中的内容。尝试一下:

@Entity
@Table(name = "t_address")
@SecondaryTables({
    @SecondaryTable(name="t_city"),
    @SecondaryTable(name="t_country")
})
public class FirstTable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String street1;
    private String street2;

    @Column(table="t_city")
    private String city;
    @Column(table="t_city")
    private String state;
    @Column(table="t_city")
    private String zipcode;

    @Column(table="t_country")
    private String country;
}