Return value of Repository.save contains only added elements of @MappedCollection List, not existing ones

[chris-unlikelyai]

I have two entities, one containing a List of the other, mapped to two tables using @MappedCollection. There's a repository for the parent:

@Table
data class ParentEntity(
  @MappedCollection val children: List<ChildEntity>,
  @Id val id: UUID? = null,
)

@Table
data class ChildEntity(@Id val id: UUID? = null)

interface EntityRepository : CrudRepository<ParentEntity, UUID>

If I have an existing entity in the database with a single child entity, loading it, adding a second child entity, and saving does the right thing in the database, but the parent entity returned by the EntityRepository.save method returns a list containing only the new child, not the original child plus the new child. Loading from the DB again returns the correct result.

Full code to reproduce below:

package springdataissue

import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.context.event.ApplicationReadyEvent
import org.springframework.boot.runApplication
import org.springframework.context.event.EventListener
import org.springframework.data.annotation.Id
import org.springframework.data.relational.core.mapping.MappedCollection
import org.springframework.data.relational.core.mapping.Table
import org.springframework.data.repository.CrudRepository
import org.springframework.data.repository.findByIdOrNull
import org.springframework.stereotype.Service
import java.util.UUID

// Schema:
//
// CREATE TABLE IF NOT EXISTS parent_entity
// (
//     id                 UUID NOT NULL DEFAULT random_uuid() PRIMARY KEY
// );
//
// CREATE TABLE IF NOT EXISTS child_entity
// (
//     id                 UUID NOT NULL DEFAULT random_uuid() PRIMARY KEY,
//     parent_entity      UUID NOT NULL REFERENCES parent_entity (id) ON DELETE CASCADE,
//     parent_entity_key  INT  NOT NULL
// );

@Table
data class ParentEntity(
  @MappedCollection val children: List<ChildEntity>,
  @Id val id: UUID? = null,
)

@Table
data class ChildEntity(@Id val id: UUID? = null)

interface EntityRepository : CrudRepository<ParentEntity, UUID>

@SpringBootApplication class SpringDataIssue

@Service
class Startup(private val entityRepository: EntityRepository) {
  @EventListener(ApplicationReadyEvent::class)
  fun onStartup() {
    val entity1 = entityRepository.save(ParentEntity(listOf(ChildEntity())))
    println("After initial save:            $entity1")

    val entity2 = entity1.copy(children = entity1.children + ChildEntity())
    println("Updated in memory:             $entity2")

    val entity3 = entityRepository.save(entity2)
    println("After saving updated entity:   $entity3")

    val entityE = entityRepository.findByIdOrNull(entity3.id!!)!!
    println("Updated entity loaded from DB: $entityE")
  }
}

fun main(args: Array<String>) {
  runApplication<SpringDataIssue>(*args)
}

// Result:
//   After initial save:            ParentEntity(children=[ChildEntity(id=542938e4-1941-40bb-852b-6b11052c5a3b)], id=c93a8ba8-0c54-4979-af25-d08f422803d7)
//   Updated in memory:             ParentEntity(children=[ChildEntity(id=542938e4-1941-40bb-852b-6b11052c5a3b), ChildEntity(id=null)], id=c93a8ba8-0c54-4979-af25-d08f422803d7)
//   After saving updated entity:   ParentEntity(children=[ChildEntity(id=a9686057-a17d-4626-b649-ab512155bfd3)], id=c93a8ba8-0c54-4979-af25-d08f422803d7)
//   Updated entity loaded from DB: ParentEntity(children=[ChildEntity(id=542938e4-1941-40bb-852b-6b11052c5a3b), ChildEntity(id=a9686057-a17d-4626-b649-ab512155bfd3)], id=c93a8ba8-0c54-4979-af25-d08f422803d7)

[schauder]

Please provide a Minimimal Reproducable Example, preferable as a Github repository. Make sure to include the database, either as an in memory database or if that is not possible using Testcontainers.

[chris-unlikelyai]

Here's a reproduceable example as a GitHub repo:

https://github.com/chris-unlikelyai/springdataissue

Clone and run mvn spring-boot:run to reproduce.

[schauder]

Analysis:

For immutable entities we gather changed entities (ChildEntity), in order to set them as values in entities (ParentEntity) referencing them. But we don't do this for unchanged (ChildEntity).

In the case of a collection of entities this leads to the effect you see, because we gather only those ChildEntity instances that actually got a new id created.