Micronaut Data: INSERT of Parent-Child relationship with autogenerated Parent ID fails cause Foreign Key not set

Question

stumbled over a problem: Within One-to-Many relationship it seems that auto-generated primary key from parent entity is NOT fed to child entity.

I've boiled down production code to following example:

1. Entities, One-to-Many relationship of Parent and Child, both with DB-generated primary key fields (Kotlin code):

import io.micronaut.data.annotation.GeneratedValue
import io.micronaut.data.annotation.Id
import io.micronaut.data.annotation.MappedEntity
import io.micronaut.data.annotation.Relation
import io.micronaut.data.annotation.Relation.Cascade.PERSIST
import io.micronaut.data.annotation.Relation.Kind.ONE_TO_MANY
import io.micronaut.data.model.naming.NamingStrategies

@MappedEntity("Parent", namingStrategy = NamingStrategies.Raw::class)
data class Parent(
  @GeneratedValue @field:Id val id: Long?,
  val name: String,
  @field:Relation(ONE_TO_MANY, mappedBy = "parent_id", cascade = [PERSIST])
  val children: List<Child>? = emptyList()
) {
  // Copy-constructor, used by ORM:
  constructor(
    name: String,
    children: List<Child>?
  ) : this(
    null,
    name,
    children
  )
}

@MappedEntity("Child", namingStrategy = NamingStrategies.Raw::class)
data class Child(
  @GeneratedValue @field:Id val id: Long?,
  val parent_id: Long?,
  val description: String,
) {
  // Copy-constructor, used by ORM:
  constructor(
    description: String,
  ) : this(null, null, description)
}

2. Repository code to persist Parent entities (Kotlin):

import io.micronaut.data.annotation.Repository
import io.micronaut.data.jdbc.annotation.JdbcRepository
import io.micronaut.data.model.query.builder.sql.Dialect.H2
import io.micronaut.data.repository.GenericRepository
import javax.transaction.Transactional

@Repository
@JdbcRepository(dialect = H2)
interface ParentRepository : GenericRepository<Parent, Long> {

  @Transactional
  fun save(parent: Parent): Parent

}

3. Now the test case. It defines DB schema and creates it with in-memory H2 database. It tries to persist a single Parent instance with one Child (written with Spock/Groovy):

import groovy.sql.Sql
import io.micronaut.test.extensions.spock.annotation.MicronautTest
import jakarta.inject.Inject
import spock.lang.Specification

import javax.sql.DataSource

@MicronautTest
class ParentRepositorySpec extends Specification {

  @Inject
  DataSource dataSource
  @Inject
  ParentRepository parentRepository

  def 'Inserting parent with single child'() {
    given:
    loadDatabaseSchemaTo(dataSource)
    def child = new Child('Single child')
    def parent = new Parent('Parent', List.of(child))

    when:
    def savedParent = parentRepository.save(parent)

    then:
    savedParent.id != null // Primary Key field gets populated
    savedParent.items[0].id != null // Same with primary key field of child item
  }

  static void loadDatabaseSchemaTo(DataSource dataSource) {
    def createTableParent = '''
CREATE TABLE `Parent` (
  `id` int(5) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) NOT NULL,
  PRIMARY KEY (`id`)
)'''
    def createTableChild = '''
CREATE TABLE `Child` (
  `id` int(5) NOT NULL AUTO_INCREMENT,
  `parent_id` int(5) NOT NULL,  -- <= Mandatory foreign key
  `description` varchar(255) NOT NULL,
  PRIMARY KEY (`id`),
  FOREIGN KEY (`parent_id`) REFERENCES `Parent` (`id`)
)'''
    new Sql(dataSource).with { sql ->
      sql.execute(createTableParent)
      sql.execute(createTableChild)
    }
  }

}

Child tables foreign key field "parent_id" is defined as MANDATORY: No Child entity without Parent.

But executing test case throws:

io.micronaut.data.exceptions.DataAccessException: SQL Error executing INSERT: SQL error executing INSERT: NULL not allowed for column "PARENT_ID"; SQL statement:
INSERT INTO `Child` (`parent_id`,`description`) VALUES (?,?) [23502-200]

Logging shows that INSERT to table "Parent" is executed followed by "Batch SQL Insert" to table "Child" ... which breaks.

So to me it seems that auto-generated primary key from Parent entity is NOT being propagated to Child entity's field "parent_id" prior to issuing the INSERT statements targeting table "child".

What is missing in my code?