hibernate的OneToMany和ManyToOne映射

1、前言

在数据库中，不管一对多或者多对一都统称为：一对多。@OneToMany和@ManyToOne指定了一对多的关系。

2、@OneToMany使用外键映射的单向一对多关联

单向关联

entity：Member

@Entity
@Setter
@Getter
@Table(name = "jei_member")
public class Member implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    @ApiModelProperty(value = "id")
    private Long id;

    @Column(name = "name")
    @ApiModelProperty(value = "姓名")
    private String name;

    @Column(name = "gender")
    @ApiModelProperty(value = "性别")
    private Integer gender;

    @OneToMany(fetch = FetchType.LAZY)
    @JoinColumn(name = "member_id", referencedColumnName = "id", insertable = false, updatable = false)
    private Set<Order> orders;

    public Set<OrderDto> getOrderDtos() {
        if (orders == null) {
            return null;
        }
        return orders.stream().map(x -> new OrderDto(x.getId(), x.getProductName(), x.getProductNum(), x.getMemberId())).collect(Collectors.toSet());
    }

}

注意：

@OneToMany(fetch = FetchType.LAZY)
@JoinColumn(name = "member_id", referencedColumnName = "id", insertable = false, updatable = false)

这里@JoinColumn()中name="member_id"指子表（MANY）中的字段， referencedColumnName = "id"指当前表(ONE)中的字段。

3、@ManyToOne使用外键映射的单向一对多关联

单向关联

entity：Order

@Entity
@Setter
@Getter
@Table(name = "jei_order")
public class Order implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    @ApiModelProperty(value = "id")
    private Long id;

    @Column(name = "product_name")
    @ApiModelProperty(value = "商品名称")
    private String productName;

    @Column(name = "product_num")
    @ApiModelProperty(value = "数量")
    private Integer productNum;

    @Column(name = "member_id")
    @ApiModelProperty(value = "会员id")
    private Long memberId;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "member_id", referencedColumnName = "id", insertable = false, updatable = false)
    private Member member;

}

注意：

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "member_id", referencedColumnName = "id", insertable = false, updatable = false)

这里的name=“member_id”， 指的当前表（MANY）的字段，referencedColumnName = "id"只能是主表（ONE）的主键id。

4、@OneToMany和@ManyToOne使用外键映射的双向一对多关联

entity：Menber

@Entity
@Setter
@Getter
@Table(name = "jei_member")
public class Member implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    @ApiModelProperty(value = "id")
    private Long id;

    @Column(name = "name")
    @ApiModelProperty(value = "姓名")
    private String name;

    @Column(name = "gender")
    @ApiModelProperty(value = "性别")
    private Integer gender;

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "member")
    private Set<Order> orders;

    public Set<OrderDto> getOrderDtos() {
        if (orders == null) {
            return null;
        }
        return orders.stream().map(x -> new OrderDto(x.getId(), x.getProductName(), x.getProductNum(), x.getMemberId(), x.getMember().getName(), x.getMember().getGender()))
                .collect(Collectors.toSet());
    }
}

注意：

@OneToMany(fetch = FetchType.LAZY, mappedBy = "member")

这里不需要@JoinColumn进行声明关联，需要用mappedBy 来确定关系由谁来维护。mappedBy="member"说明在两个关联的实体Bean中，member这一端是关系的拥有者。

entity：Order

@Entity
@Setter
@Getter
@Table(name = "jei_order")
public class Order implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    @ApiModelProperty(value = "id")
    private Long id;

    @Column(name = "product_name")
    @ApiModelProperty(value = "商品名称")
    private String productName;

    @Column(name = "product_num")
    @ApiModelProperty(value = "数量")
    private Integer productNum;

    @Column(name = "member_id")
    @ApiModelProperty(value = "会员id")
    private Long memberId;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "member_id", referencedColumnName = "id",insertable = false,updatable = false)
    private Member member;

}

注意：

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "member_id", referencedColumnName = "id", insertable = false, updatable = false)

这里的name=“member_id”， 指的当前表（MANY）的字段，referencedColumnName = "id"只能是主表（ONE）的主键id。

总结：
1、@JoinColumn(name = "many_id", referencedColumnName = "one_id", insertable = false, updatable = false)
2、在一对多的情况下，一定要注意死循环的问题，实体类中使用@Setter、@Getter。@data双向关联的情况下，会出现栈溢出。