Kotlin - How to get a property from an object in collection - kotlin

Question

I have never worked before with Kotlin so I have a newbie question. I am working with an existing codebase, so I am wondering about a few things. I see that there is a function getDepartmentById which looks like this:
fun getDepartmentById(ctx: Ctx, params: JsonObject): Either<Failure, FlatResp> =
getOneByIdFlattened(ctx, params.right(), getDepartmentByIdSql(ctx), flattenOne = flattenerToType(MainAccessType.DEPARTMENT))
fun getDepartmentById(ctx: Ctx, id: Long): Either<Failure, FlatResp> =
getDepartmentById(ctx, jsonObject("id" to id))
Calling that function returns either Failure or FlatResp. From what I can see in the code FlatResponse is typealias for Map<MainAccessType, Entities>.
The function getOneByIdFlattened looks like this:
fun getOneByIdFlattened(ctx: Ctx,
params: Either<Long, JsonObject>,
statement: String,
rowConverter: (Row) -> Map<String, Any?> = ::mapFromDbNames,
grouper: (List<Map<String, Any?>>) -> List<Map<String, Any?>> = ::identity,
flattenOne: (List<Map<String, Any?>>) -> FlatResp
): Either<Failure, FlatResp> =
either.eager {
val id = when (params) {
is Either.Left -> Either.Right(params.value)
is Either.Right -> params.value.idL?.right()
?: Failure.JsonError(SErr(GlowErrs.MISSING_ID, "You must provide id")).left()
}.bind()
val dbDataList: List<Map<String, Any?>> = doQuery(ctx, statement, mapOf(
"courierIds" to ctx.user.courierIds,
"id" to id,
"count" to 1,
"offset" to 0,
"departmentIds" to ctx.userDepartments,
"customerIds" to ctx.user.customerIds
),
rowConverter, false
).bind()
val result = flattenOne(grouper(dbDataList))
addUpdatedAtEpoch(result)
}
I wonder how can I get from a FlatResp a property of an object, that looks like this:
So, for example if I want to get just name from this object what would be the best way to do this?
Also, I wonder why is this function returning a collection, and not just a single object when it should get a single row by id from DB?
This is the sql function:
private fun getDepartmentByIdSql(ctx: Ctx) =
"""select ${createSelectFields(departmentKeys)}
from department dept
where dept.id = :id
${
when (ctx.user.role) {
UserRoles.ADMIN -> ""
else -> "and dept.id = any (:departmentIds) "
}
}"""

Answer 1

So there are a lot of things wrong in the snippet provided. Given Snippets:
1
fun getDepartmentById(ctx: Ctx, params: JsonObject): Either<Failure, FlatResp> =
getOneByIdFlattened(ctx, params.right(), getDepartmentByIdSql(ctx), flattenOne = flattenerToType(MainAccessType.DEPARTMENT))
2
fun getDepartmentById(ctx: Ctx, id: Long): Either<Failure, FlatResp> =
getDepartmentById(ctx, jsonObject("id" to id))
3
private fun getDepartmentByIdSql(ctx: Ctx) =
"""select ${createSelectFields(departmentKeys)}
from department dept
where dept.id = :id
${
when (ctx.user.role) {
UserRoles.ADMIN -> ""
else -> "and dept.id = any (:departmentIds) "
}
}"""
4
fun getOneByIdFlattened(ctx: Ctx,
params: Either<Long, JsonObject>,
statement: String,
rowConverter: (Row) -> Map<String, Any?> = ::mapFromDbNames,
grouper: (List<Map<String, Any?>>) -> List<Map<String, Any?>> = ::identity,
flattenOne: (List<Map<String, Any?>>) -> FlatResp
): Either<Failure, FlatResp> =
either.eager {
val id = when (params) {
is Either.Left -> Either.Right(params.value)
is Either.Right -> params.value.idL?.right()
?: Failure.JsonError(SErr(GlowErrs.MISSING_ID, "You must provide id")).left()
}.bind()
val dbDataList: List<Map<String, Any?>> = doQuery(ctx, statement, mapOf(
"courierIds" to ctx.user.courierIds,
"id" to id,
"count" to 1,
"offset" to 0,
"departmentIds" to ctx.userDepartments,
"customerIds" to ctx.user.customerIds
),
rowConverter, false
).bind()
val result = flattenOne(grouper(dbDataList))
addUpdatedAtEpoch(result)
}
Issues:
in snippet 2, jsonObject should be JsonObject(..)
I have no idea what the following lines do :
//snippet1:
flattenOne = flattenerToType(MainAccessType.DEPARTMENT))
//snipper 4
either.eager {...block...}
addUpdatedAtEpoch(result)
doQuery(ctx, statement, mapOf(..)
//snippet3
UserRoles.ADMIN -> ""
"""select ${createSelectFields(departmentKeys)}
They are all probably some extension functions or util files made by your company or from some famous libraries like anko orsplitties . plus these are mixes with function calls of your own class, like createSelectFields or ctx.user.courierIds. also if i have to guess, then this seems like an unusual way of performing some operation on an sql dB
based on just code completion by android studio, i have been able to figure out the classes as following:
class Entities
typealias FlatResp = Map<MainAccessType, Entities>
class Ctx
sealed class Either<A,B>(val a:A?, val b:B?){
val value:A? = null
class Left<A>(val aa:A):Either<A,A>(aa,aa)
class Right<B>(val bv:B):Either<B,B>(bb,bb)
}
class Failure
class Row
class JsonObject(val pair:Pair<String,Long>):JSONObject(){
fun right():Either<Long,JsonObject>{
}
}
class jsonObject()
enum class MainAccessType{DEPARTMENT}
fun getDepartmentById(ctx: Ctx, params: JsonObject): Either<Failure, FlatResp> {
return getOneByIdFlattened(
ctx,
params.right(),
getDepartmentByIdSql(ctx),
flattenOne = flattenerToType(MainAccessType.DEPARTMENT))
}
fun getDepartmentById(ctx: Ctx, id: Long): Either<Failure, FlatResp> {
return getDepartmentById(ctx, JsonObject("id" to id))
}
fun getOneByIdFlattened(ctx: Ctx,
params: Either<Long, JsonObject>,
statement: String,
rowConverter: (Row) -> Map<String, Any?> = ::mapFromDbNames,
grouper: (List<Map<String, Any?>>) -> List<Map<String, Any?>> = ::identity,
flattenOne: (List<Map<String, Any?>>) -> FlatResp
): Either<Failure, FlatResp> {
return either.eager {
val id = when (params) {
is Either.Left -> Either.Right(params.value)
is Either.Right -> params.value.idL?.right()
?: Failure.JsonError(SErr(GlowErrs.MISSING_ID, "You must provide id")).left()
}.bind()
val dbDataList: List<Map<String, Any?>> = doQuery(ctx, statement, mapOf(
"courierIds" to ctx.user.courierIds,
"id" to id,
"count" to 1,
"offset" to 0,
"departmentIds" to ctx.userDepartments,
"customerIds" to ctx.user.customerIds
),
rowConverter, false
).bind()
val result = flattenOne(grouper(dbDataList))
addUpdatedAtEpoch(result)
}
}
private fun getDepartmentByIdSql(ctx: Ctx) =
"""select ${createSelectFields(departmentKeys)}
from department dept
where dept.id = :id
${
when (ctx.user.role) {
UserRoles.ADMIN -> ""
else -> "and dept.id = any (:departmentIds) "
}
}"""
fun mapFromDbNames(row:Row): Map<String,Any?>{
}
fun identity(param : List<Map<String, Any?>>): List<Map<String, Any?>>{
}
This is still not correct and has a lots of red lines in it. but what you can do is keep this as a starter in a separate file, compare and fix the code accordingly and then maybe we can tell what would be a better way:
replace inline functions (fun xyz(...) = someValue ) to block functions. (alt+enter in windows, cmd+n in mac)
instead of typeAlias, use map directly
::something means a function is passed as parameter . its similar to how we pass runnables in java 8, but even more shorthand. you can do ctrl+click( for mac its cmd+click) on that function and goto that function to check what its params are, what its return type are. do the same for various classes/ extension fucntions, variables too. this will help the most
instead of passing something into something which is being passed into another thing (like val bot = Robot(Petrol("5Litres") ) ) , split into different lines to make it understandable ( val amount = "5litres"; val equipment = Petrol(amount) ; val bot = Robot(equipment) )
try to not use 3rd party library/ replace with your own understandable code.
repeat steps 1-5
Hope this gives someplace to start. kotlin is a beautiful language but is also very easy to make unreadable.

Answer 2

Mapping Map values
I wonder how can I get from a FlatResp a property of an object, that looks like this:
So, for example if I want to get just name from this object what would be the best way to do this?
TL;DR
Without data to work with, here's my guess:
val extractedNames: Map<Long, String?> = destinationDepartment
.mapValues { (_, userData: Map<String, Any?>) ->
when (val name = userData["name"]) {
is String -> name
else -> null
}
}
println(extractedNames)
// {1=Bergen, 2=Cindy, 3=Dave}
Intro
Kotlin is great for manipulating collections. For a more general of how to work with collections in Kotlin, I think the docs are really clear Collection transformation operations#Map.
Let's see how that works for this example. You want to extract a specific element, so for that we can use map().
From your screenshot it looks like this is a Map<Long, Map>, where the value is a Map<String, Any?>. I'll assume you want to change the Map<Long, Map> to a Map<Long, String>, where the key is the database ID and the value is user's name.
Test data
So I've got something to test with, I made a new Map:
val destinationDepartment: Map<Long, Map<String, Any?>> =
mapOf(
1L to mapOf(
"id" to 1,
"name" to "Bergen",
"createdAt" to LocalDateTime.now(),
"updatedAt" to LocalDateTime.now(),
),
2L to mapOf(
"id" to 2,
"name" to "Cindy",
"createdAt" to LocalDateTime.now(),
"updatedAt" to LocalDateTime.now(),
),
3L to mapOf(
"id" to 3,
"name" to "Dave",
"createdAt" to LocalDateTime.now(),
"updatedAt" to LocalDateTime.now(),
),
)
Basic noop
First, set up the basics. A Map can be converted to a list of Entries. When we call map(), it will iterates over each Entry, and applies a lambda - which is something we must write. In this instance, the lambda receives the key and value of the Map, and must return a new value.
Aside: the Java equivalent is map.entrySet().stream().map(...)...
Here, the lambda just returns a pair (created with to).
val extractedNames = destinationDepartment
.map { (id: Long, userData: Map<String, Any?>) ->
id to userData
}
println(extractedNames)
// Output: [(1, {id=1, name=Bergen, createdAt=2021-08-19T11:00:07.447660, updatedAt=2021-08-19T11:00:07.449969}),
// (2, {id=2, name=Cindy, createdAt=2021-08-19T11:00:07.463813, updatedAt=2021-08-19T11:00:07.463845}),
// (3, {id=3, name=Dave, createdAt=2021-08-19T11:00:07.463875, updatedAt=2021-08-19T11:00:07.463890})]
Pretty boring! But now we're set up for the next step - extracting name from userData: Map<String, Any?>.
Extracting name
val extractedNames = destinationDepartment
.map { (id: Long, userData: Map<String, Any?>) ->
val name = userData["name"]
id to name
}
println(extractedNames)
// Output: [(1, Bergen), (2, Cindy), (3, Dave)]
Now there's loads of ways to improve this. Making sure that name is a String, not Any?, filtering out blank or null names, mapping to DTOs, sorting. Again, the Kotlin documentation would be a good start. I'll start by listing one really good improvement.
Converting List<Pair<>> to Map<>
If you look at the type of val extractedNames, you'll see that it's a list, not a map.
val extractedNames: List<Pair<Long, Any?>> = ...
That's because the lambda we wrote in the map() function is returning a Pair<Long, String>. Kotlin doesn't know that this is still considered a Map. We can convert any List<Pair<>> back to a map with toMap()
toMap()
val extractedNames: Map<Long, Any?> = destinationDepartment
.map { (id: Long, userData: Map<String, Any?>) ->
val name = userData["name"]
id to name
}
.toMap() // convert List<Pair<>> to a Map<>
println(extractedNames)
// Output: {1=Bergen, 2=Cindy, 3=Dave}
But this is also not great. Why is id: Long in the lambda if we're not using it? Because we're only extracting the name from userData, we're only mapping the values of the Map. We don't need id: Long at all. Fortunately Kotlin has another useful method: mapValues() - and it returns a Map<>, so we can drop the toMap(). Let's use it.
mapValues()
val extractedNames: Map<Long, Any?> = destinationDepartment
.mapValues { (id: Long, userData: Map<String, Any?>) ->
val name = userData["name"]
id to name
}
println(extractedNames)
// {1=(1, Bergen), 2=(2, Cindy), 3=(3, Dave)}
Umm weird. Why are the ids in the values? That's because the mapValues() lambda should return the new value, and in our lambda we're returning both the id and name - oops! Let's only return the name.
Fixing mapValues()
val extractedNames: Map<Long, Any?> = destinationDepartment
.mapValues { (_, userData: Map<String, Any?>) ->
userData["name"]
}
println(extractedNames)
// {1=(1, Bergen), 2=(2, Cindy), 3=(3, Dave)}
Better! Because id is not used, an underscore can be used instead
Aside: Note that the lambda doesn't have a return. Read Returning a value from a lambda expression for an explanation.