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)
}

@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.

👍