jpa - 一次选择现有和非现有的连接实体

Question

我有一个连接表，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;
}

如何列出所有现有和不存在ExchangeRate的给定sourceCurrency？

例如，

1. 共有三种货币。A、B 和 C。
2. 已经有一个从 A 到 B 的 ExhangeRate

用下面的方法，如果sourceCurrency是A，

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

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();
}

对于给定 sourceCurrency 的非现有 ExchangeRates

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