H2 嵌入式数据库外键和引用

Question

So, I have a homework where I need to implement two entities - Account and Transaction Futher on I need to be able to create a transaction between two accounts (scenario: only 2 account involved)

I also need to find out table's structure for Transaction and would appreciate a help for Transaction table creation script.

My question is: I need to find out how to reference the foreign keys and relationship between these two tables as for now it is a bit unclear for me.

Logic is next: I was thinking and maybe an Account would have two List<Transaction> as in:

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

therefore: in Transaction class would be:

@ManyToOne referencing the transactionMadeList

private Account emitter; 

@ManyToOne referencing the transactionReceivedList

private Account receptor;

So here are my classes

Account.class

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;

}

Transaction.class

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;
    }

}

For now that's it.

Thank You in advance! :)

Answer 1

You are thinking in the right direction. Here is how your schema will look like:

The only two things I noticed you could do better:

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

Change this:

private LocalDateTime created;

To this:

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;

will become this:

@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;

If you get famous LazyInitializationException , try wrapping it around @Transactional . It might complicate it a bit, however, in the long run, will save you some server resources, because you won't be loading all account's transaction when they are not even needed.

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);
}