Fix concat

[AndreiKingsley]

concat removes key column entirely (name and values)

[AndreiKingsley]

Maybe it's useful to add GroupBy.origin: DataFrame? that returns original dataframe if it was created via DataFrame.groupBy()

[Jolanrensen]

I think this is the intended behavior. The key of the group is something temporary and usually consists of columns already in the DF.
We are working on a way to access the group keys from aggregate though (#662), maybe that can be a nice alternative.

The original DataFrame can be retrieved using concat (albeit with a different order perhaps).

[AndreiKingsley]

Ok, anyway new concat is needed for the purpose I described.

[AndreiKingsley]

This's my current solution
https://github.com/Kotlin/kandy/blob/main/kandy-api/src/main/kotlin/org/jetbrains/kotlinx/kandy/dsl/internal/concatFixed.kt

[Jolanrensen]

maybe a concatWithKeys() would be a nice addition?

[koperagen]

I think it won't hurt to make do it by default. One might say that df.groupBy { expr { } } is a shortcut for df.add() { }.groupBy {}

[Jolanrensen]

if we do it by default, then we would get duplicate columns, because the key columns are often in the groups as well

[koperagen]

Andrey's implementation only adds "new" columns (or so i understood)

[Jolanrensen]

But then, what qualifies as "new"?

• groupBy { expr { myCol } }, yes
• but groupBy { myCol + 1 }?
• or groupBy { myCol named "other" }

I think we should be careful here

[Jolanrensen]

There's also the case where a user creates a new expr column with a duplicate name that should still be kept, so my suggestion is the following:
Create a concatWithKeys() that will add all key columns to the front of the groups regardless of whether they were in the DF already. Avoid naming clashes by using the ColumnNameGenerator, for instance with DynamicDataFrameBuilder.

Something like:

internal fun GroupBy<*, *>.concatWithKeys(): DataFrame<*> =
    mapToFrames {
        DynamicDataFrameBuilder()
            .apply {
                for (column in group.columns()) {
                    add(column)
                }
                val rowsCount = group.rowsCount()
                for ((name, value) in key.toMap()) {
                    add(List(rowsCount) { value }.toColumn(name))
                }
            }
            .toDataFrame()
            .moveToLeft { takeLast(key.count()) }
    }.concat()

[Jolanrensen]

Alternatively, what's arguably a lot simpler, we could just explode the groups column. Like:

internal fun GroupBy<*, *>.concatWithKeys(): DataFrame<*> =
    toDataFrame().explode { groups }

This will generate extra key values where necessary and keep the grouped columns in a column group, avoiding any potential name clashes :).