一次选择现有和非现有的连接实体

Question

我的连接表看起来像这样。

EXCHANGE_RATE
------------------------------
ID                  BIGINT  PK
SOURCE_CURRENCY_ID  BIGINT  FK
TARGET_CURRENCY_ID  BIGINT  FK

映射的ExchangeRate.java

public class ExchangeRate {

    protected ExchangeRage() {
        this(null, null);
    }

    // for SELECT NEW
    public ExchangeRate(Currency sourceCurrency, Currency targetCurrency) {
        this.sourceCurrency = sourceCurrency;
        this.targetCurrency = targetCurrency;
    }

    @Id private Long id;
    @ManyToOne private Currency sourceCurrency;
    @ManyToOne private Currency targetCurrency;
}

如何为给定的ExchangeRate列出所有现有和不存在的sourceCurrency？

例如，

1. 有三种货币。 A，B和C.
2. 已经有从A到B的ExhangeRate

使用以下方法，如果sourceCurrency为A，

public List<ExchangeRate> listExchangeable(Currency sourceCurrency) {
    //
}

我如何获得以下列表？

ExchangeRate {
    id:             0
    sourceCurrency: A
    targetCurrency: B
},
ExchangeRate {
    id:             NULL
    sourceCurrency: A
    targetCurrency: C
}

Answer 1

必填查询

// get all exchange rates for a source currency
public List<ExchangeRate> listExchangeable(Currency sourceCurrency) {
  Query qry = em.createQuery("select r from ExchangeRate r where 
                                 r.sourceCurrency = :sourceCurrency");
  qry.setParameter("sourceCurrency", sourceCurrency);
  return qry.getResultList();
}

表示给定sourceCurrency的非现有ExchangeRates

Query qry = em.createQuery("select r from ExchangeRate r where 
                                 r.sourceCurrency != :sourceCurrency");