Kalıtım Stratejileri — Üç Ödeme Türü, Kaç Tablo?

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// Only shares fields: not an entity, no table, nothing can reference it.
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@MappedSuperclass
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abstract class Audited {
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    @Id
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    @GeneratedValue(strategy = GenerationType.SEQUENCE)
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    protected Long id;
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    protected Instant createdAt = Instant.now();
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    public Long id() {
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        return id;
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    }
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}

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@Entity
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@Table(name = "payments")
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@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
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@DiscriminatorColumn(name = "kind", length = 16)
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abstract class Payment extends Audited {
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    protected long customerId;
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    protected BigDecimal amount;
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    protected Payment() {
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    }
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    protected Payment(long customerId, BigDecimal amount) {
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        this.customerId = customerId;
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        this.amount = amount;
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    }
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    abstract String describe();
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}

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@Entity
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@DiscriminatorValue("CARD")      // fixed: renaming the class will not orphan old rows
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class CardPayment extends Payment {
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    private String cardLast4;
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    protected CardPayment() {
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    }
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    CardPayment(long customerId, BigDecimal amount, String cardLast4) {
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        super(customerId, amount);
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        this.cardLast4 = Objects.requireNonNull(cardLast4);
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    }
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    @Override
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    String describe() {
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        return "Kart •••• " + cardLast4;
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    }
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}
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@Entity
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@DiscriminatorValue("TRANSFER")
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class TransferPayment extends Payment {
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    private String iban;
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    protected TransferPayment() {
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    }
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    TransferPayment(long customerId, BigDecimal amount, String iban) {
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        super(customerId, amount);
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        this.iban = Objects.requireNonNull(iban);
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    }
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    @Override
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    String describe() {
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        return "Havale " + iban;
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    }
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}

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create table payments (
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    id          bigint primary key,
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    kind        varchar(16)    not null,
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    created_at  timestamp      not null,
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    customer_id bigint         not null,
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    amount      numeric(19, 2) not null,
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    card_last4  char(4),            -- nullable: empty on transfer rows
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    iban        varchar(34),        -- nullable: empty on card rows
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    -- each kind still has to fill its own column
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    constraint payments_kind_columns check (
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        (kind = 'CARD'     and card_last4 is not null) or
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        (kind = 'TRANSFER' and iban       is not null)
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    )
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);
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create sequence payment_seq increment by 50;   -- Hibernate 6 default name for the Payment root

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interface PaymentRepository extends JpaRepository<Payment, Long> {
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    // Polymorphic: card and transfer rows come back as their own classes, no JOIN.
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    List<Payment> findByCustomerIdOrderByCreatedAtDesc(long customerId);
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    // One kind: Hibernate adds "where kind = 'CARD'" by itself.
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    @Query("select c from CardPayment c where c.customerId = :customerId")
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    List<CardPayment> findCards(long customerId);
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}

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@DataJpaTest
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class PaymentMappingTest {
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    @Autowired PaymentRepository payments;
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    @Autowired JdbcTemplate jdbc;
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    @Test
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    void aCardPaymentIsOneRowWithTheTransferColumnsEmpty() {
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        payments.saveAndFlush(new CardPayment(7L, new BigDecimal("250.00"), "4242"));
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        Map<String, Object> row = jdbc.queryForMap("select kind, card_last4, iban from payments");
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        assertThat(row).containsEntry("kind", "CARD").containsEntry("card_last4", "4242");
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        assertThat(row.get("iban")).isNull();
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    }
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}