spring data rest kotlin associations POST - kotlin

Question

I followed the tutorial http://www.baeldung.com/spring-data-rest-relationships.
I also observed that I can create the association directly by providing the link to the relationship.
curl -i -X POST -H "Content-Type:application/json" -d '{"name":"My Library"}' http://localhost:8080/libraries
curl -i -X POST -d '{"title":"Books", "library":"http://localhost:8080/libraries/1"}' -H "Content-Type:application/json" http://localhost:8080/books
This works fine in Java and also in Kotlin when using a regular class.
However, if I use a data class in Kotlin, I get the following error
2018-04-26 14:13:43.730 ERROR 79256 --- [nio-8080-exec-2] b.e.h.RestResponseEntityExceptionHandler : org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot construct instance of com.baeldung.models.Library (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('http://localhost:8080/libraries/1'); nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of com.baeldung.models.Library (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('http://localhost:8080/libraries/1') at [Source: (org.apache.catalina.connector.CoyoteInputStream); line: 1, column: 29] (through reference chain: com.baeldung.models.Book["library"])
I do have the relevant kotlin-spring, kotlin-jpa and kotlin-noarg plugins in my project.
Code is here https://github.com/vijaysl/spring-data-rest

Answer 1

Try adding #JsonCreator(mode = JsonCreator.Mode.DISABLED) annotation on primary constructor. No need to disable the com.fasterxml.jackson.module:jackson-module-kotlin.
Explanation:
Kotlin Jackson module implies your default constructor is the JSON creator (see KotlinValueInstantiator class).
Therefore, Spring Data REST does not apply its bean deserializer modifier (that is supposed to load a bean by URI) because bean properties mappings are not used for creator properties (constructor params).
KotlinValueInstantiator tries to deserialize constructor params using standard deserializers and instantiators and this leads to the error you mentioned.
Possible solution:
Since koltin-jpa module adds a default empty constructor for JPA, you can instruct Jackson not to use the JSON creator but the default empty constructor by explicitly disabling it.
Example:
#Entity
class Book #JsonCreator(mode = JsonCreator.Mode.DISABLED) constructor(
#ManyToMany
val libraries: ModifiableList<Library> = ArrayList(),
): AbstractPersistable<Long>(), Identifiable<Long>

Answer 2

Kotlin data classes are pretty strict. It's telling you, basically, it can't construct your POKO and it's listing some of the ways it tries. One of them is with a String constructor. Others are through private field manipulation (which is the way it's been done normally).
Data classes in kotlin, if they have fields declared as private val name:String translate to (in java) private final String name; It can't assign to a final field (which is dirty to try to assign to a private field, but impossible when it's final; the JVM won't allow it) and there are no getName() or setName() functions which can be used as another method of hydration.
Some options:
Declare your variables are var instead of val. private var name:String is java equiavalent to private String name which will use field based (dirty) hydration.
include a specific kotlin dependency for kotlin that fixes this issue: compile("com.fasterxml.jackson.module:jackson-module-kotlin") have a look at this project
example kotlin class that should work for you:
import org.springframework.hateoas.Identifiable
import java.time.LocalDate
import javax.persistence.*
import javax.validation.constraints.*
#Entity
data class Employee(#Pattern(regexp = "[A-Za-z0-9]+")
#Size(min = 6, max = 32)
val name: String,
#Email
#NotNull
val email: String?,
#PastOrPresent
val hireDate: LocalDate = LocalDate.now(),
#OneToMany(mappedBy = "employee", cascade = [CascadeType.ALL])
val forms:List<Form> = listOf(),
#OneToMany(mappedBy = "employee", cascade = [CascadeType.ALL])
val reports:List<Report> = listOf(),
#Id #GeneratedValue( strategy = GenerationType.IDENTITY) private val id: Long? = null): Identifiable<Long> {
override fun getId() = id
constructor(name:String): this(name,"$name#foo.com")
}

Answer 3

With kotlin all Ok.
Just replace "data class" to "class".
Jackson don't find empty constructor in "data class". And use other deserializator... not Uri....