java - H2 嵌入式数据库外键和引用

Question

因此，我有一项作业需要实现两个实体 - 账户和交易进一步我需要能够在两个账户之间创建交易(场景:只涉及 2 个账户)

我还需要找出 Transaction 表的结构，希望对 Transaction 表创建脚本有帮助。

我的问题是:我需要找出如何引用这两个表之间的外键和关系，因为现在我有点不清楚。

接下来是逻辑:我在想，也许是一个Account会有两个 List<Transaction>如:

private List<Transaction> transactionsMade; 
private List<Transaction> transactionsReceived.

因此:在 Transaction类将是:

@ManyToOne引用 transactionMadeList

private Account emitter; 

@ManyToOne引用 transactionReceivedList

private Account receptor;

这是我的类(class)

账户类

public class Account {

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

    @NotNull
    private String holder;

    @NotNull
    private Integer balance;

    @OneToMany(mappedBy = "emitter",fetch = FetchType.EAGER,cascade = CascadeType.ALL)
    private List<Transaction> transactionsMade;

    @OneToMany(mappedBy = "receptor",fetch = FetchType.EAGER,cascade = CascadeType.ALL)
    private List<Transaction> transactionsReceived;

}

事务类

public class Transaction {

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

    @NotNull
    private Integer amount;

    @NotNull
    private LocalDateTime created;

    @ManyToOne
    @JoinColumn(name = "transactionEmitter")
    private Account emitter;

    @ManyToOne
    @JoinColumn(name = "transactionReceptor")
    private Account receptor;

    public Transaction(Integer amount, Account emitter, Account receptor){
        this.created = LocalDateTime.now();
        this.amount = amount;
        this.emitter = emitter;
        this.receptor = receptor;
    }

}

现在就是这样。

提前谢谢您! :)

Answer 1

您的思考方向是正确的。您的架构如下所示:

我注意到只有两件事你可以做得更好:

1) Use Instant instead of LocalDateTime, so it will be stored in UTC time.

改变这个:

private LocalDateTime created;

对此:

private Instant created;

2) Do not use EAGER initialization:

@OneToMany(mappedBy = "emitter",fetch = FetchType.EAGER,cascade = CascadeType.ALL)
private List<Transaction> transactionsMade;

@OneToMany(mappedBy = "receptor",fetch = FetchType.EAGER,cascade = CascadeType.ALL)
private List<Transaction> transactionsReceived;

会变成这样:

@OneToMany(mappedBy = "emitter",fetch = FetchType.LAZY,cascade = CascadeType.ALL)
private List<Transaction> transactionsMade;

@OneToMany(mappedBy = "receptor",fetch = FetchType.LAZY,cascade = CascadeType.ALL)
private List<Transaction> transactionsReceived;

如果您遇到著名的 LazyInitializationException，请尝试将其包裹在 @Transactional 中。这可能会使事情变得有点复杂，但是，从长远来看，将为您节省一些服务器资源，因为您不会在甚至不需要时加载所有帐户的交易。

3) Create a repository to modularize your functionality, just like this one:

@Repository
public interface TransactionRepository extends JpaRepository<Transaction, Long> {
    Collection<Transaction> findAllByEmitter(Account emitter);
    Collection<Transaction> findAllByReceptor(Account receptor);
}