How to find the updated fields between a payload and an entity fetched from DB and create an object having fields with updated values and rest Null - kotlin

Question

Given an update request for a record in DB, I have to find a difference between the payload and existing data in DB then create a new Object which has updated fields with Payload values and rest as Null.
I have created a function which gives me a list of field names which were updated, But I'm unable to create a new object which has values for only these updated fields.The problem is that the function uses "field: Field in cpayload.javaClass.declaredFields" which is kind of generic so I'm unable to set these fields.
fun findupdatedFieldsList(cpayload: Customer, cEntity: Customer): List<String> {
// var customerToPublish = Customer()
val updatedFieldsList: MutableList<String>
updatedFieldsList = ArrayList()
for (field: Field in cpayload.javaClass.declaredFields) {
field.isAccessible = true
val value1 = field.get(cpayload).toString()
val value2 = field.get(cEntity).toString()
!Objects.equals(value1, value2).apply {
if (this) {
// customerToPublish.birthDate=field.get(cpayload).toString()
updatedFieldsList.add(field.name)
}
}
}
return updatedFieldsList
}
#Entity
#Table
data class Customer(
#Id
val partyKey: UUID,
var preferredName: String?,
var givenName: String?,
var lastName: String?,
var middleName: String?,
var emailAddress: String,
var mobileNumber: String,
val birthDate: String?,
val loginOnRegister: Boolean,
var gender: Gender?,
var placeOfBirth: String?,
var createdDate: LocalDateTime = LocalDateTime.now(),
var updatedDate: LocalDateTime = LocalDateTime.now()
)
Desired Output
val customer = Customer(
preferredName = Updated name,
partyKey = partyKey.value,
givenName = Updated name,
lastName = null,
middleName = null,
emailAddress = Updated email,
mobileNumber = null,
birthDate = null,
gender = null,
placeOfBirth = null
)

Answer 1

I was able to construct a solution using Kotlin's reflect. It is generic and can be applied to any Kotlin class that have primary constructor. Unfortunately it won't work with Java classes
You would need to add kotlin-reflect package to your build tool config, e.g. for Gradle:
implementation 'org.jetbrains.kotlin:kotlin-reflect:XXXXXX'
First we will build a function to extract updated properties. Please take a note that we also need to extract properties that are mandatory (non-nullable and without default). We add them to a map of propertyName -> propertyValue:
fun Map<String?, KParameter>.isOptional(name: String) = this[name]?.isOptional ?: false
fun <T : Any> findUpdatedProperties(payload: T, entity: T): Map<String, Any?> {
val ctorParams = payload::class.primaryConstructor!!.parameters.associateBy { it.name }
return payload::class.memberProperties.map { property ->
val payloadValue = property.call(payload)
val entityValue = property.call(entity)
if (!Objects.equals(payloadValue, entityValue) || (!ctorParams.isOptional(property.name))) {
property.name to payloadValue
} else {
null
}
}
.filterNotNull()
.toMap()
}
Then we call this function and construct a new instance of provided class:
fun <T : Any> constructCustomerDiff(clazz: KClass<T>, payload: T, entity: T): T {
val ctor = clazz.primaryConstructor!!
val params = ctor.parameters
val updatedProperties = findUpdatedProperties(payload, entity)
val values = params.map { it to updatedProperties[it.name] }.toMap()
return ctor.callBy(values)
}
Take a note that missing primary constructor will throw NullPointerException because of use of !!.
We could call this funcion as constructCustomerDiff(Customer::class, payload, entity), but we can do better with reified types:
inline fun <reified T : Any> constructCustomerDiff(payload: T, entity: T): T {
return constructCustomerDiff(T::class, payload, entity)
}
Now we can use this function in convenient Kotlin style:
val id = UUID.randomUUID()
val payload = Customer(
partyKey = id,
preferredName = "newName",
givenName = "givenName"
)
val entity = Customer(
partyKey = id,
preferredName = "oldName",
givenName = "givenName" // this is the same as in payload
)
val x = constructCustomerDiff(payload, entity)
assert(x.partyKey == id && x.givenName == null || x.preferredName == "newName")