JPA中@OneToOne 与 @OneToMany 的用法

Spring工程中，创建实体对象时，可以通过JPA的@Entity标识实体与数据库表的对应关系，@Column标识数据库字段。其中还有标识两个实体间关系的注解：@OneToOne、@OneToMany、@ManyToOne和@ManyToMany，分别标识一对一、一对多、多对一和多对多。在此，简单记录下@OneToOne和@OneToMany的使用。其中还会涉及注解@JoinColumn，其修饰的字段为关系维护字段。

@OneToOne

　　@OneToOne与@JoinColumn结合使用，表示会在源实体（Source Entity，即声明该注解的实体类中创建外键，进行级联），以ShipOrderInfo和ShipOrderRecords为例。

ShipOrderInfo类，会在ship_order_info表中创建ship_order_records的外键；
ShipOrderRecords类，mappedBy = "shipOrderInfo"，表示由ShipOrderInfo类的shipOrderRecordsList维护外键；
保存ShipOrderInfo对象的时候，会先将ShipOrderRecords对象保存，然后设置ShipOrderInfo对象外键，并保存。即先保存targetEntity，设置外键后，再保存sourceEntity。

@OneToMany

　　@OneToMany与@JoinColumn结合使用，会在目标实体（targetEntity指定的类中创建外键）。保存的时候，先保存sourceEntity，设置外键后，再保存targetEntity。如ShipOrderRecords 和 ShipOrdedLogs是一对多的情况。

@Data
@Entity(name = "ShipOrderInfo")
@Table(name = "ship_order_info")
public class ShipOrderInfo extends AuditModel implements Serializable {private static final long serialVersionUID = -3984010638987217501L;@Id@GeneratedValue(strategy = GenerationType.AUTO,generator = "pg-uuid")@GenericGenerator(name = "pg-uuid",strategy = "uuid2",parameters = @org.hibernate.annotations.Parameter(name = "uuid_gen_strategy_class",value = "com.decathlon.shiphub.common.util.PostgreSQLUUIDGenerationStrategy"))@Column(name = "id", columnDefinition = "uuid default uuid_generate_v4()")private UUID id;@Column(name = "ship_group_id")@NotEmpty(message = "ship_group_id must not be empty")private String shipGroupId;@Column(name = "order_id")private String orderId;@Column(name = "tracking_number")private String trackingNumber;@Column(name = "estimated_delivery_time", columnDefinition = "timestamptz")private Instant estimatedDeliveryTime;@Column(name = "real_delivery_time", columnDefinition = "timestamptz")private Instant realDeliveryTime;@Column(name = "order_paytime", columnDefinition = "timestamptz")private Instant orderPaytime;@Column(name = "sports_user_id")private String sportsUserId;@Column(name = "client_channel")private Integer clientChannel;@Column(name = "label_url")private String labelURL;@Column(name = "b2c_order_id")private String b2cOrderId;@Column(name = "order_latest_status")private Integer orderLatestStatus;@Column(name = "data_status", columnDefinition = "int2 default 0")private Byte dataStatus;@Column(name = "carrier_id")private UUID carrierId;@Column(name = "location_info_id")private UUID locationInfoId;@Column(name = "delivery_type")private String deliveryType;@Column(name = "contact_phone")private String contactPhone;@Column(name = "order_platform")private String orderPlatform;@Column(name = "click_collect_destination")private String clickCollectDestination;@Column(name = "order_sequence")private String orderSequence;@OneToOne(mappedBy = "shipOrderInfo", cascade = CascadeType.REFRESH, fetch = FetchType.LAZY)@JsonManagedReferenceprivate List<ShipOrderRecords> shipOrderRecordsList;
}

@Data
@Entity(name = "ShipOrderRecords")
@Table(name = "ship_order_records")
public class ShipOrderRecords extends AuditModel implements Serializable {private static final long serialVersionUID = -6873931504419257515L;@Id@GeneratedValue(strategy = GenerationType.AUTO,generator = "pg-uuid")@GenericGenerator(name = "pg-uuid",strategy = "uuid2",parameters = @org.hibernate.annotations.Parameter(name = "uuid_gen_strategy_class",value = "com.decathlon.shiphub.common.util.PostgreSQLUUIDGenerationStrategy"))@Column(name = "id", columnDefinition = "uuid default uuid_generate_v4()")private UUID id;@Column(name = "ship_status", columnDefinition = "int2 default 0")private int shipStatus;@Column(name = "reason_code")private String reasonCode;@Column(name = "comment")private String comment;@Column(name = "data_status", columnDefinition = "int2 default 0")private Byte dataStatus;@OneToMany(mappedBy = "shipOrderRecords", cascade = CascadeType.REFRESH, fetch = FetchType.LAZY)@JsonManagedReferenceprivate List<ShipOrderLogs> shipOrderLogsList;@OneToOne(cascade = CascadeType.REFRESH)@JoinColumn(name = "order_info_id")@JsonBackReferenceprivate ShipOrderInfo shipOrderInfo;
}

@Data
@Entity(name = "ShipOrderLogs")
@Table(name = "ship_order_logs")
public class ShipOrderLogs extends AuditModel implements Serializable {private static final long serialVersionUID = 7671064435799278019L;@Id@GeneratedValue(strategy = GenerationType.AUTO,generator = "pg-uuid")@GenericGenerator(name = "pg-uuid",strategy = "uuid2",parameters = @org.hibernate.annotations.Parameter(name = "uuid_gen_strategy_class",value = "com.decathlon.shiphub.common.util.PostgreSQLUUIDGenerationStrategy"))@Column(name = "id", columnDefinition = "uuid default uuid_generate_v4()")private UUID id;@Column(name = "request_body", length = 3000)private String requestBody;@Column(name = "response_body", length = 3000)private String responseBody;@Column(name = "log_desc")private String logDesc;@ManyToOne(cascade = CascadeType.REFRESH)@JoinColumn(name = "order_records_id")@JsonBackReferenceprivate ShipOrderRecords shipOrderRecords;
}

需要注意的是：

　　（1）多对多情况，可以不用这种注解的方式，可以通过中间表维护级联关系（比如A和B表多对多，C表只包含2列A和B的主键映射，维护多对多关系），操作可能更方便。

　　（2）这种级联谨慎使用，在查询等操作的时候，查A实体，也会把级联的B或C实体也查出来，但业务只需要A的信息，所以可能增加数据库负担。