Is there a nice way to extend an existing column type? For example, if I have a

value class Id(id: String)

I’d like to have a column

Column<Id>

that wraps a

varchar(12)

.

I'm trying to do exactly this as well. For TextColumnType and VarCharColumnType you can extend those classes and add whatever custom functionality you'd like. for instance,

class AlwaysBlue : VarCharColumnType(colLength = 4) {
  override fun valueFromDB(value: Any): Any = "blue"
  override fun valueToDB(value: Any?): Any? = "blue"
  override fun notNullValueToDB(value: Any): Any = "blue"
  override fun nonNullValueToString(value: Any): String = "blue"
}

After creating your custom class, you can attach it to the Table type using an extension function

fun <T> Table.alwaysBlue(name: String): Column<String> = 
    registerColumn(name, AlwaysBlue())

Unfortunately, most of the ColumnType implementations are final, and can't be extended this way. To get around this, I'd recommend the delegate pattern.

open class DelegatingColumnType(private val delegate: ColumnType, nullable: Boolean = false) : ColumnType(nullable) {
    override fun sqlType(): String = delegate.sqlType()

    override fun valueFromDB(value: Any): Any = delegate.valueFromDB(value)

    override fun valueToString(value: Any?): String = delegate.valueToString(value)

    override fun valueToDB(value: Any?): Any? = delegate.valueToDB(value)

    override fun notNullValueToDB(value: Any): Any = delegate.notNullValueToDB(value)

    override fun nonNullValueToString(value: Any): String = delegate.nonNullValueToString(value)

    override fun readObject(rs: ResultSet, index: Int): Any? = delegate.readObject(rs, index)

    override fun setParameter(stmt: PreparedStatement, index: Int, value: Any?) =
        delegate.setParameter(stmt, index, value)
}

Using this you can extend any column type like so

class AlwaysFour : DelegatingColumnType(IntegerColumnType()) {
  override fun valueFromDB(value: Any): Any = 4
  override fun valueToDB(value: Any?): Any? = 4
  override fun notNullValueToDB(value: Any): Any = 4
  override fun nonNullValueToString(value: Any): String = 4
}

I've had some luck using a single column CompositeColumn to act as a mapper to my business objects. I'll add the snippet here once I'm at my desk.