whats the best way to convert from a list of objec...

# 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?

[Wesley Acheson]

@Ubi I was doing multiple levels of grouping.

[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.

[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.

[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.

[Wesley Acheson]

Thanks

[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