Combining/merging data classes in Kotlin - kotlin

Question

Is there a way to merge kotlin data classes without specifying all the properties?
data class MyDataClass(val prop1: String, val prop2: Int, ...//many props)
with a function with the following signature:
fun merge(left: MyDataClass, right: MyDataClass): MyDataClass
where this function checks each property on both classes and where they are different uses the left parameter to create a new MyDataClass.
Is this possible possible using kotlin-reflect, or some other means?
EDIT: more clarity
Here is a better description of what i want to be able to do
data class Bob(
val name: String?,
val age: Int?,
val remoteId: String?,
val id: String)
#Test
fun bob(){
val original = Bob(id = "local_id", name = null, age = null, remoteId = null)
val withName = original.copy(name = "Ben")
val withAge = original.copy(age = 1)
val withRemoteId = original.copy(remoteId = "remote_id")
//TODO: merge without accessing all properties
// val result =
assertThat(result).isEqualTo(Bob(id = "local_id", name = "Ben", age=1, remoteId = "remote_id"))
}

Answer 1

If you want to copy values from the right when values in the left are null then you can do the following:
inline infix fun <reified T : Any> T.merge(other: T): T {
val propertiesByName = T::class.declaredMemberProperties.associateBy { it.name }
val primaryConstructor = T::class.primaryConstructor
?: throw IllegalArgumentException("merge type must have a primary constructor")
val args = primaryConstructor.parameters.associateWith { parameter ->
val property = propertiesByName[parameter.name]
?: throw IllegalStateException("no declared member property found with name '${parameter.name}'")
(property.get(this) ?: property.get(other))
}
return primaryConstructor.callBy(args)
}
Usage:
data class MyDataClass(val prop1: String?, val prop2: Int?)
val a = MyDataClass(null, 1)
val b = MyDataClass("b", 2)
val c = a merge b // MyDataClass(prop1=b, prop2=1)

Answer 2

A class-specific way to combine data classes when we can define the fields we want to combine would be:
data class SomeData(val dataA: Int?, val dataB: String?, val dataC: Boolean?) {
fun combine(newData: SomeData): SomeData {
//Let values of new data replace corresponding values of this instance, otherwise fall back on the current values.
return this.copy(dataA = newData.dataA ?: dataA,
dataB = newData.dataB ?: dataB,
dataC = newData.dataC ?: dataC)
}
}

Answer 3

#mfulton26's solution merges properties that are part of primary constructor only. I have extended that to support all properties
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.associate { parameter ->
val property = nameToProperty[parameter.name]!!
parameter to (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
}

Answer 4

Your requirements are exactly the same as copying the left value:
fun merge(left: MyDataClass, right: MyDataClass) = left.copy()
Perhaps one of use isn't properly understanding the other. Please elaborate if this isn't what you want.
Note that since right isn't used, you could make it a vararg and "merge" as many as you like :)
fun merge(left: MyDataClass, vararg right: MyDataClass) = left.copy()
val totallyNewData = merge(data1, data2, data3, data4, ...)
EDIT
Classes in Kotlin don't keep track of their deltas. Think of what you get as you're going through this process. After the first change you have
current = Bob("Ben", null, null, "local_id")
next = Bob(null, 1, null, "local_id")
How is it supposed to know that you want next to apply the change to age but not name? If you're just updating based on nullability,
#mfulton has a good answer. Otherwise you need to provide the information yourself.

Answer 5

infix fun <T : Any> T.merge(mapping: KProperty1<T, *>.() -> Any?): T {
//data class always has primary constructor ---v
val constructor = this::class.primaryConstructor!!
//calculate the property order
val order = constructor.parameters.mapIndexed { index, it -> it.name to index }
.associate { it };
// merge properties
#Suppress("UNCHECKED_CAST")
val merged = (this::class as KClass<T>).declaredMemberProperties
.sortedWith(compareBy{ order[it.name]})
.map { it.mapping() }
.toTypedArray()
return constructor.call(*merged);
}
Edit
infix fun <T : Any> T.merge(right: T): T {
val left = this;
return left merge mapping# {
// v--- implement your own merge strategy
return#mapping this.get(left) ?: this.get(right);
};
}
Example
val original = Bob(id = "local_id", name = null, age = null, remoteId = null)
val withName = original.copy(name = "Ben")
val withAge = original.copy(age = 1)
val withRemoteId = original.copy(remoteId = "remote_id")
val result = withName merge withAge merge withRemoteId;