How to use another variable name or How to flatten entity in JPA Projection for nested object - spring-boot

Question

I'm making an api for querying nested entity with Spring Data JPA Projection.
My Code
The Entity:
#Entity
class User {
#Id
var userId: String
var name: String
var age: Int
#OneToOne
#JoinColumn(name = "userId")
var address: Address
}
#Entity
class Address {
var userId: String
var street: String
var city: String
var country: String
}
The Repository:
interface UserView {
val name: String
val address: AddressView
interface AddressView {
val city: String
val country: String
}
}
#Repository
interface UserRepository : JPARepository<User, String> {
fun findAll(): List<UserView>
}
Expected Response
{
"name": "example",
"city": "example-city",
"country": "example-country"
}
My code produces
{
"name": "example",
"address": {
"city": "example-city",
"country": "example-country"
}
}
I Tried
I tried another view to flatten object:
interface UserView {
val name: String
val addressCity: String
val addressCountry: String
}
But this case, the variable naming is too complicate.
I want to solve this problem with projection. How can I solve this problem?

Answer 1

In JPA, you can do this using #NamedNativeQuery only:
#NamedNativeQuery(
name = "getUser",
query = "SELECT u.name, a.city, a.country FROM User u and Address a where a.userId = u.id ", resultClass=UserView .class)
#Entity
class User {
...
}
For Reference hibernate-named-query

Answer 2

Try this:
data class UserView(name: String, city: String, country: String)
#Repository
interface UserRepository : JPARepository<User, String> {
#Query(value = "select new your.pkg.UserView(u.name, u.address.city, u.address.country) from User u")
fun findAllFlat(): List<UserView>
}

Answer 3

You can use #Value and combine more fields or even access the fields of objects.
From the spring-data-rest documentation:
You can create a projection that combines the two data fields in the preceding example together, as follows:
#Projection(name = "virtual", types = { Person.class })
public interface VirtualProjection {
#Value("#{target.firstName} #{target.lastName}")
String getFullName();
}
Spring’s #Value annotation lets you plug in a SpEL expression that takes the target object and splices together its firstName and lastName attributes to render a read-only fullName.
Flattening also works for me:
#Value("#{target.document.title}")
String getDocumentTitle();