JPA #OneToMany doesn't save parent ID - spring-boot

Question

i develop API using Spring Boot and JPA
I found a case in Bidirectional OneToMany relationship, my parent ID doesn't save in DB when requested by JSON payload like this :
{
"name": "Rice",
"description": "Lorem ipsum dolor sit amet",
"variants": [
{
"amount": 1,
"unit": "kg",
"price": 60000
}
]
}
Here is my Model :
Product.kt
data class Product(
#Id
#GeneratedValue
var id: Long = 0,
var name: String = "",
var photo: String = "",
var description: String = "",
#OneToMany(mappedBy = "product", cascade = [CascadeType.ALL])
#JsonBackReference
var variants: MutableList<Variant> = mutableListOf()
)
Variant.kt
#Entity
data class Variant(
#Id
#GeneratedValue
var id: Long = 0,
var amount: Int = 0,
var unit: String = "",
var price: Double = 0.0,
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = "product_id")
#JsonManagedReference
var product: Product? = null
)
Is there any other configuration need to be applied?
Thank you

Answer 1

The problem is, that during JSON deserialization the back reference from Variant to Product is not set.
And you define
#OneToMany(mappedBy = "product", cascade = [CascadeType.ALL])
#JsonBackReference
var variants: MutableList<Variant> = mutableListOf()
mappedBy says that the back reference is the owning side and therefor in charge for setting the foreign key.
You have two options:
Either you set the product reference in Variant before saving the Product
or you remove the back reference from Variant to Product and remove the mappedBy = "product" from the variants collection.