Spring mongo aggregate nested elements null - spring

Question

I have 3 collections: attributes, products and product_attributes. I am trying to retrieve product attributes with aggregation with nested attribute and product in it. But product and attribute always null
service method:
fun findById(id: String): Mono<ProductAttribute> {
val idField = "_id"
val productId = "productId"
val attributeId = "attributeId"
val attributesDb = "attributes"
val productsDb = "products"
val attributeName = "attribute"
val productName = "product"
fun lookup(from: String, localField: String, name: String) = LookupOperation.newLookup()
.from(from)
.localField(localField)
.foreignField(idField)
.`as`(name)
fun match(id: String) = Aggregation.match(Criteria.where(idField).`is`(ObjectId(id)))
fun unwind(field: String) = Aggregation.unwind(field)
val aggregation = Aggregation.newAggregation(
match(id),
lookup(productsDb, productId, productName),
unwind(productName),
lookup(attributesDb, attributeId, attributeName),
unwind(attributeName)
).withOptions(AggregationOptions.builder().allowDiskUse(true).build())
return operations.aggregate(aggregation, "product_attributes", ProductAttribute::class.java)
.last()
.doOnError { throwable -> logger.error("Failed to get productAttribute", throwable) }
}
ProductAttribute.kt
#Document(collection = "product_attributes")
data class ProductAttribute(
#Id
#JsonProperty("_id")
val id: String? = ObjectId().toHexString(),
#get:Transient #Value("null") val product: Product?,
#get:Transient #Value("null") val attribute: Attribute?,
val attributeId: String,
val productId: String,
val name: String,
val quantity: Int,
val photos: List<String>
) : Serializable
Response:
{
"data": {
"productAttributeId": {
"id": "62a3bff787418b6f837e9150",
"product": null,
"attribute": null,
"name": "string",
"quantity": 1,
"photos": [
"string"
]
}
}
}

Answer 1

As a workaround I have ended up with this result:
I made aggregate method with generic type. It will aggregate to Any type and then map to my specific type.
inline fun <reified T> aggregate(
aggregation: Aggregation,
collectionName: String,
operations: ReactiveMongoOperations,
objectMapper: ObjectMapper
): Flux<T> =
operations.aggregate(aggregation, collectionName, Any::class.java)
.map { data -> objectMapper.convertValue(data, T::class.java) }
I think there is a way to implement custom converter. I will think about this later. I will update answer on success