Kotlin - 按地图列表分组

Question

I have a fieldList variable.

val fieldList: List<MutableMap<String, String>>

// fieldList Data :

[ {
  "field_id" : "1",
  "section_id" : "1",
  "section_name" : "section1",
  "field_name" : "something_1"
}, {
  "field_id" : "2",
  "section_id" : "1",
  "section_name" : "section1",
  "field_name" : "something_2"
}, {
  "field_id" : "3",
  "section_id" : "2",
  "section_name" : "section2",
  "field_name" : "something_3"
}, {
  "field_id" : "4",
  "section_id" : "3",
  "section_name" : "section3",
  "field_name" : "something_4"
} ]

And I want to group by section_id.

The results should be as follows:

val result: List<MutableMap<String, Any>>

// result Data :

[
   {
      "section_id": "1",
      "section_name": "section1",
      "field": [
         {
            "id": “1”,
            "name": "something_1"
         },
         {
            "id": “2”,
            "name": "something_2"
         }
      ]
   },
   {
      "section_id": "2",
      "section_name": "section2",
      "field": [
         {
            "id": “3”,
            "name": "something_3"
         }
      ]
   },
   .
   .
   .
]

What is the most idiomatic way of doing this in Kotlin?

I have an ugly looking working version in Java, but I am quite sure Kotlin has a nice way of doing it..

it's just that I am not finding it so far !

Any idea ?

Thanks

Answer 1

Assuming we are guaranteed that the data is correct and we don't have to validate it, so:

• all fields always exist,
• section_name is always the same for a specific section_id .

This is how you can do this:

val result = fieldList.groupBy(
    keySelector = { it["section_id"]!! to it["section_name"]!! },
    valueTransform = {
        mutableMapOf(
            "id" to it["field_id"]!!,
            "name" to it["field_name"]!!,
        )
    }
).map { (section, fields) ->
    mutableMapOf(
        "section_id" to section.first,
        "section_name" to section.second,
        "field" to fields
    )
}

However, I suggest not using maps and lists, but proper data classes. Using a Map to store known properties and using Any to store either String or List is just very inconvenient to use and error-prone.

Answer 2

Another way:

val newList = originalList.groupBy { it["section_id"] }.values
    .map {
        mapOf(
            "section_id" to it[0]["section_id"]!!,
            "section_name" to it[0]["section_name"]!!,
            "field" to it.map { mapOf("id" to it["field_id"], "name" to it["field_name"]) }
        )
    }

Playground

Also, as broot mentioned, prefer using data classes instead of such maps.