I'm trying to figure out how to add a group_concat...

# komapper

[dave08]

I'm trying to figure out how to add a group_concat function... I know I must have the types wrong though... how do I do this @Toshihiro Nakamura?

inline fun <reified T : Any> groupConcat(vararg columns: ColumnExpression<T, String>) : ColumnExpression<T, String> {
        val operands = columns.map { Operand.Column(it) }
        return columnExpression(T::class, String::class, { this as T }, "group_concat", operands.toList()) {
            append("GROUP_CONCAT(")
            operands.dropLast(1).forEach { visit(it); append(", ") }
            visit(operands.last())
            append(")")
        }
    }

[dave08]

There seems to be two variants of the columnExpression function, and I don't really understand the first arguments too much...

[Toshihiro Nakamura]

Try the following code:

fun <T: Any> groupConcat(vararg columns: ColumnExpression<T, String>) : ColumnExpression<T, String> {
    if (columns.isEmpty()) error("empty columns")
    val operands = columns.map { Operand.Column(it) }
    return columnExpression(columns.first(), "group_concat", operands.toList()) {
        append("GROUP_CONCAT(")
        operands.dropLast(1).forEach { visit(it); append(", ") }
        visit(operands.last())
        append(")")
    }
}

[dave08]

I get this:|

Type mismatch.
Required:
ColumnExpression<TypeVariable(T), String>
Found:
PropertyMetamodel<SomeEntity, SomeId, Int>

[dave08]

when I try to use it in my select statement...

[dave08]

It seems that this works:

fun <E: Any, I: Any> groupConcat(vararg columns: ColumnExpression<E, I>) : ColumnExpression<E, I> {
        if (columns.isEmpty()) error("empty columns")
        val operands = columns.map { Operand.Column(it) }
        return columnExpression(columns.first(), "group_concat", operands.toList()) {
            append("GROUP_CONCAT(")
            operands.dropLast(1).forEach { visit(it); append(", ") }
            visit(operands.last())
            append(")")
        }
    }

[dave08]

But that seems to mean that only columns with the same interior exterior types can be provided...

[Toshihiro Nakamura]

OK. This code should work:

fun groupConcat(vararg columns: ColumnExpression<*, *>) : ColumnExpression<String, String> {
    if (columns.isEmpty()) error("empty columns")
    val operands = columns.map { Operand.Column(it) }
    return columnExpression(String::class, String::class, {it}, "group_concat", operands.toList()) {
        append("GROUP_CONCAT(")
        operands.dropLast(1).forEach { visit(it); append(", ") }
        visit(operands.last())
        append(")")
    }
}

[dave08]

Yup, that worked, thanks!

This could be a nice inclusion for the next version...

It would also be amazing if you could add kdocs for those functions to explain what those params are and how to use them... they're not internal, since they're in the official docs, and it's not really clear just by looking at them.