whats the best way to convert from a list of objects (think database results) to a nested object set...

# announcements

[Wesley Acheson]

whats the best way to convert from a list of objects (think database results) to a nested object set. For instance a JDBC results set that used GROUP BY day, status which would give me a list with the same day more than once and the same status more than once which returned a list of

data class DayStatusCount(val day:LocalDate, val status:Status, val count:Long)

If I wanted to convert to a Map<LocalDate, Map<Status, Long>> or anythings similar to that. I've been creating mutable maps.

val rawData: MutableMap<LocalDate, Map<Status, Long> = HashMap()
        data.forEach {
            rawData
                    .getOrPut(it.day) { HashMap() }
                    .getOrPut(it.status) { HashMap() }
// More as per step
        }

Sorry thats not the real code. but anyway convert from a list with duplicates to nested sets.

[Ubi]

import java.time.LocalDate

data class DayStatusCount(val day: LocalDate, val status: String, val count: Long)


val rawData: MutableMap<LocalDate, Map<String, Long>> = HashMap()


val data = listOf(
        DayStatusCount(day = LocalDate.of(2001, 1, 1), status = "TODO", count = 1L),
        DayStatusCount(day = LocalDate.of(2001, 1, 1), status = "IN_PROGRESS", count = 2L),
        DayStatusCount(day = LocalDate.of(2001, 1, 2), status = "TODO", count = 1L),
        DayStatusCount(day = LocalDate.of(2001, 1, 3), status = "TODO", count = 1L),
        DayStatusCount(day = LocalDate.of(2001, 1, 4), status = "TODO", count = 1L)
)

val groupdData: Map<LocalDate, List<DayStatusCount>> = data.groupBy { it -> it.day }

[Ubi]

do you just want to group by data? Like the above example?

[Ruckus]

You could try something like

val mapped = results.groupBy { it.day }
    .mapValues { (_, value) ->
        value.groupBy(
            { it.status },
            { it.count }
        )
    }

[Wesley Acheson]

@Ubi I was doing multiple levels of grouping.

[Ruckus]

If you're not crazy into all the functional stuff, maybe

fun List<DayStatusCount>.group(): Map<LocalDate, Map<Status, List<Long>>> {
    val mapped = mutableMapOf<LocalDate, MutableMap<Status, MutableList<Long>>>()
    forEach {
        mapped.getOrPut(it.day) { mutableMapOf() }
            .getOrPut(it.status) { mutableListOf() } += it.count
    }
    return mapped
}

(This one will probably perform and scale better.)

[Wesley Acheson]

Yeah thats similar to my initial approach. I find it hard to think in the functional stuff at times. except you use mutableMapOf() rather than HashMap() Also I like the array syntax you use. which I'm still getting my head around. Not array exactly more like associative array.

[Ruckus]

Agreed. Also always going for a "one-liner" tends towards non-readability and bad performance when taken too far.

[Wesley Acheson]

Its not the one line I try to go for. but generally avoiding mutation.

[Wesley Acheson]

but in this case its mutation during construction I can live with that.

[Ruckus]

I tend to stick to mutableMapOf unless I specifically need a HashMap . (Which isn't all that common, usually I just need a map.)

[Wesley Acheson]

Java habits. only thing about mutableMapOf() when you have nothing there is no "Of" I'd prefer it called mutableMap() but thats just trivial.

[Ruckus]

Indeed, and if this is in a separate method, you can always return it as a Map instead of MutableMap to clean up the API for the user.

[Wesley Acheson]

Thanks

[Ruckus]

As far as the Of suffix goes, if I'm not passing anything in, I usually have to add the generic types, so it still reads pretty well to me (in my head this reads as "mutable map of local date to (mutable map of status to long)")

[Ubi]

How about:

val mapped: Map<LocalDate, Map<String, Long>> = data.groupBy { it.day }
        .mapValues { (_, value) -> value.map { it.status to it.count } }
        .mapValues { (_, values) -> values.toMap() }

[Wesley Acheson]

Looks good.

[Ubi]

Its based on https://stackoverflow.com/questions/53433108/kotlin-from-a-list-of-maps-to-a-map-grouped-by-key