Merge two data classes in Kotlin

merge.kt

import kotlin.reflect.full.declaredMemberProperties
import kotlin.reflect.full.primaryConstructor

/**
 * Merge two data classes
 *
 * The resulting data class will contain:
 * - all fields of `other` which are non null
 * - the fields of `this` for the fields which are null in `other`
 *
 * The function is immutable, the original data classes are not changed 
 * and a new data class instance is returned.
 *
 * Example usage:
 *
 *     val a = MyDataClass(...)
 *     val b = MyDataClass(...)
 *     val c = a merge b
 */
infix inline fun <reified T : Any> T.merge(other: T): T {
    val nameToProperty = T::class.declaredMemberProperties.associateBy { it.name }
    val primaryConstructor = T::class.primaryConstructor!!
    val args = primaryConstructor.parameters.associateWith { parameter ->
        val property = nameToProperty[parameter.name]!!

        (property.get(other) ?: property.get(this))
    }
    return primaryConstructor.callBy(args)
}

I use your util with
implementation 'org.jetbrains.kotlin:kotlin-reflect:1.3.41'
But can not receive any merged field. :(

I just tested (also using Kotlin 1.3.41), all seems to work fine. How are you using it?

Thanks for your response.
Here is my config:

• kotlin_version = '1.3.40'
• classpath 'com.android.tools.build:gradle:3.4.1'
• implementation 'org.jetbrains.kotlin:kotlin-reflect:1.3.41'

REPL:

import com.example.test_only.ProductModel
import com.example.test_only.merge

var apple1 = ProductModel("RED", "APPLE", "SMALL")
var apple2 = ProductModel("GREEN", "APPLE", "BIG")
var apple3 = apple1.merge(apple2)
println("$apple1")
println("$apple2")
println("$apple3")
println("-----")
apple1 = ProductModel("RED", "APPLE", "SMALL")
apple2 = ProductModel("GREEN", "APPLE", "BIG")
apple3 = apple2.merge(apple1)
println("$apple1")
println("$apple2")
println("$apple3")

Output

ProductModel(color=RED, fruit=APPLE, size=SMALL)
ProductModel(color=GREEN, fruit=APPLE, size=BIG)
ProductModel(color=GREEN, fruit=APPLE, size=BIG)
-----
ProductModel(color=RED, fruit=APPLE, size=SMALL)
ProductModel(color=GREEN, fruit=APPLE, size=BIG)
ProductModel(color=RED, fruit=APPLE, size=SMALL)

And what does ProductModel look like?

I have tried 2 model class:
package com.example.test_only

data class ProductModel(
var color: String?,
var fruit: String?,
var size: String?
)

and

app: build.gradle

androidExtensions {
    experimental = true
}

import android.os.Parcelable
import kotlinx.android.parcel.Parcelize

@parcelize
data class ProductModel(
var color: String?,
var fruit: String?,
var size: String?
) : Parcelable

Thanks, now I can reproduce your results. The output looks as expected to me: apple1 and apple2 stay untouched, and apple3 is the merge result of apple1 and apple2. What would you expect?

What output would you expect for apple3 in your two examples?

Oh finally I got your ideal.

• Stay untouched with apple1 and apple2.
• Merged object is new object called apple3.

import com.example.test_only.ProductModel
import com.example.test_only.merge

var appleOld = ProductModel(null, "APPLE", null, 1)
var appleNew = ProductModel("GREEN", null, "BIG", 2)
var appleMerged = appleOld.merge(appleNew)
println("$appleOld")
println("$appleNew")
println("$appleMerged")
println("-----")

appleOld = ProductModel(null, "APPLE", null, 1)
appleNew = ProductModel("GREEN", null, "BIG", 2)
appleMerged = appleNew.merge(appleOld)
println("$appleOld")
println("$appleNew")
println("$appleMerged")

Output

ProductModel(color=null, fruit=APPLE, size=null, num=1)
ProductModel(color=GREEN, fruit=null, size=BIG, num=2)
ProductModel(color=GREEN, fruit=APPLE, size=BIG, num=2)

ProductModel(color=null, fruit=APPLE, size=null, num=1)
ProductModel(color=GREEN, fruit=null, size=BIG, num=2)
ProductModel(color=GREEN, fruit=APPLE, size=BIG, num=1)

Thanks for your response.
You save me all day.!

Yes indeed :)

Will update update the explanatory comments

We already did it. 👍

hoiulj

I'm merging objects which has inherited properties so I ended up using the following snippet.

Also .associateWith simplifies the loop a little.

import kotlin.reflect.full.memberProperties
import kotlin.reflect.full.primaryConstructor

inline infix fun <reified T : Any> T.merge(other: T): T {
    val nameToProperty = T::class.memberProperties.associateBy { it.name }
    val primaryConstructor = T::class.primaryConstructor!!
    val args = primaryConstructor.parameters.associateWith { parameter ->
        val property = nameToProperty[parameter.name]!!
        property.get(other) ?: property.get(this)
    }
    return primaryConstructor.callBy(args)
}

I was hoping this would work with let's call them deep classes?
data class Some(val a: String?, val b: String?)
data class Other(val some: Some?, val c: String?, val d: String?)

when i do Other(Some("a", null), "c", null).merge(Other(Some(null, "b"), null, "d))
I'd expect to get Other(Some("a", "b"), "c", "d")

It's indeed a shallow merge. Deep merge would be interesting.

@josdejong did you have any solution about. A deep merge would be interesting?

@e-g-hategan I have the same requirement.

i m Doing Single level marge using This Code

inline infix fun <reified T : Any> T.merge(other: T): T {
    val nameToProperty = T::class.declaredMemberProperties.associateBy { it.name }
    val primaryConstructor = T::class.primaryConstructor!!
    val args =
        primaryConstructor.parameters.associateWith { parameter ->
            val property = nameToProperty[parameter.name]!!
            (property.get(other) ?: property.get(this))
        }
    val mergedObject = primaryConstructor.callBy(args)
    nameToProperty.values.forEach { it ->
        run {
            val property = it as KMutableProperty<*>
            val value = property.javaGetter!!.invoke(other) ?: property.javaGetter!!.invoke(this)
            property.javaSetter!!.invoke(mergedObject, value)
        }
    }
    return mergedObject
} 

How can I add extra condition, for example:

• The resulting data class will contain:
• • all fields of other which are non null or zero
• • the fields of this for the fields which are null or zero in other

or

• The resulting data class will contain:
• • all fields of other which are non null or Double 0.0
• • the fields of this for the fields which are null or Double 0.0 in other

Which line I have to put this kind of conditions?

@talatkuyuk this logic captured in the line:

parameter to (property.get(other) ?: property.get(this))

Right now the behavior is: take the property from other, but if that is null, take the property from from this.

You can adjust the exact behavior of when to pick a property from either other or this as you like, i.e. add an extra conditional check to pick other only when it is non null and not zero (when it is a numeric value).

@josdejong Thank you very much. I understand it, but I am new in Kotlin. I am not familiar with to operator. I didn't write any inflix function yet. Just I needed this.

• The resulting data class will contain:
• • all fields of other which are non null or non Double with value 0.0
• • the fields of this for the fields which are null or Double with value 0.0 in other

I tried to make it, I am stuck.

Something like this - just change the exact logic to meet your needs (and think through what logic you need for Double, Float, Int. etc, there may be a smart way to check whether any numeric type is zero, I don't know):

    infix inline fun <reified T : Any> T.mergeIgnoreZero(other: T): T {
        val nameToProperty = T::class.declaredMemberProperties.associateBy { it.name }
        val primaryConstructor = T::class.primaryConstructor!!
        val args = primaryConstructor.parameters.associateWith { parameter ->
            val property = nameToProperty[parameter.name]!!
            val valueOther = property.get(other)
            val valueThis = property.get(this)

            if (valueOther is Double && valueOther == 0.0) {
                valueThis
            } else {
                valueOther ?: valueThis
            }
        }
        return primaryConstructor.callBy(args)
    }

@josdejong Thank you very much. It worked.

👍