如何使用 Exposed 在两个字段上连接两个表？

Question

我有以下数据库表：

// CurrenciesTable
object CurrenciesTable : Table("currencies") {
    val symbol = varchar("symbol", 48)
    val name = varchar("name", 48)
    override val primaryKey = PrimaryKey(symbol)
}


// OrdersTable
object OrdersTable : IntIdTable("orders") {
    val baseCurrency = varchar("base_currency", 48)
    val counterCurrency = varchar("counter_currency", 48)
    val price = decimal("price", DECIMAL_PRECISION, DECIMAL_SCALE)
    val createdAtEpochSecond = long("created_at_epoch_second")
}

在 OrdersTable 中，我必须要引用 CurrenciesTable 的字段：

baseCurrency
counterCurrency

我想 select 来自 OrdersTable 的记录，并在两个字段上将它们与 CurrenciesTable 连接起来。所以我得到了每种货币的符号和名称。

这是我的 DSL 查询，仅加入 baseCurrency 字段。

// Exposed DSL
OrdersTable.join(CurrenciesTable, JoinType.INNER, OrdersTable.baseCurrency, CurrenciesTable.symbol)
            .selectAll()
            .forEach {
                // Getting OrdersTable record data
                it[OrdersTable.id].value
                it[OrdersTable.price]
                it[OrdersTable.createdAtEpochSecond]

                // Getting CurrenciesTable record data (for baseCurrency only)
                it[CurrenciesTable.symbol]
                it[CurrenciesTable.name]
            }

我尝试按如下方式进行第二次连接：

// Exposed DSL
OrdersTable.join(CurrenciesTable, JoinType.INNER, OrdersTable.baseCurrency, CurrenciesTable.symbol)
           .join(CurrenciesTable, JoinType.INNER, OrdersTable.counterCurrency, CurrenciesTable.symbol)

但是，我得到以下异常。

Caused by: java.sql.SQLSyntaxErrorException: Not unique table/alias: 'currencies'

Answer 1

尝试为 CurrenciesTable 添加别名:

OrdersTable.innerJoin(CurrenciesTable.alias("baseCurrency"), { OrdersTable.baseCurrency }, { CurrenciesTable.symbol })
           .innerJoin(CurrenciesTable, { OrdersTable.counterCurrency }, { CurrenciesTable.symbol })

如何使用 Exposed 在两个字段上连接两个表？

How to join two tables on two fields using Exposed?

kotlin

kotlin-exposed