Keep relationships of entity returned by saveReturning

[davidsonnabend]

Feature description

As discussed in #2694 an entity returned by saveReturning() are created based on the result of executed insert query only. Already present relationships of the entity will be removed/ignored.

To match the behavior of the "default" save() method, the relationships of the entity should be returned.

Taking the following entity (from this example application):

@Serdeable
@MappedEntity
data class Event(
    @field:Id
    @field:GeneratedValue
    val id: Long? = null,

    @field:Version
    val version: Long? = null,

    @field:GeneratedValue
    val createdAt: LocalDateTime? = null,

    val property: String,

    @Relation(value = Relation.Kind.MANY_TO_ONE)
    val child: Child? = null,
)

And the following snippet:

val child = childRepository.save(Child(value = "child3"))
val event = eventRepository.saveReturning(Event(property = "test", child = child))

I would expect event.child to be not null.