How to use JPA ManyToMany bidirectional relation with Kotlin - spring-boot

Question

I have two classes which have bidirectional ManyToMany relations in a Spring Boot application. When I would like to fetch my entities, they start recursively looping, and I get a stackoverflow exception. These is my implementation.
#Entity
#Table(name = "route")
data class Route(
#Column(name = "uid")
#Type(type = "pg-uuid")
#Id
var uid: UUID,
var image: String,
#Column(name = "rate_id")
var rate_id: UUID,
#ManyToMany(cascade = [CascadeType.ALL], fetch = FetchType.LAZY)
#JoinTable(name = "ach",
joinColumns = [JoinColumn(name = "route_id", referencedColumnName = "uid")],
inverseJoinColumns = [JoinColumn(name = "athlete_id", referencedColumnName = "uid")])
var athletes: List<Athlete> = mutableListOf())
#Entity
#Table(name = "athlete")
data class Athlete(
#Column(name = "uid")
#Type(type = "pg-uuid")
#Id
var uid: UUID,
var email: String,
var image: String,
#ManyToMany(mappedBy = "athletes")
var routes: List<Route> = mutableListOf())
I understand that the problem is that both of my list attribute is in the constructor. However I would like to have the list attributes in the response. I have seen solutions where the toString method was overwritten to create a json string. I would prefer to return an object instead of a jsonString. Is there a way to implement the above problem with or without dataclasses? Please give some example if there is a way.

Answer 1

Please notice that this answer is solution for Kotlin data classes with ManyToMany bidirectional relation.
#ManyToMany(cascade = [CascadeType.ALL], fetch = FetchType.LAZY)
#JoinTable(name = "ach",
joinColumns = [JoinColumn(name = "route_id", referencedColumnName = "uid")],
inverseJoinColumns = [JoinColumn(name = "athlete_id", referencedColumnName = "uid")])
#JsonIgnoreProperties("routes")
var athletes: List<Athlete> = mutableListOf())
#ManyToMany(mappedBy = "athletes")
#JsonIgnoreProperties("athletes")
var routes: List<Route> = mutableListOf())
With adding the #JsonIgnoreProperties, you can avoid the recursive loop.

Answer 2

For me the above solution didn't work. Instead I had to override the equals and hashCode methods to avoid the recursion like so:
#Entity
#Table(name = "route")
data class Route(
#Column(name = "uid")
#Type(type = "pg-uuid")
#Id
var uid: UUID,
var image: String,
#Column(name = "rate_id")
var rate_id: UUID,
#ManyToMany(cascade = [CascadeType.ALL], fetch = FetchType.LAZY)
#JoinTable(name = "ach",
joinColumns = [JoinColumn(name = "route_id", referencedColumnName = "uid")],
inverseJoinColumns = [JoinColumn(name = "athlete_id", referencedColumnName = "uid")])
var athletes: List<Athlete> = mutableListOf()) {
override fun equals(other: Any?): Boolean {
if (this === other) return true
if (javaClass != other?.javaClass) return false
other as Route
if (uid != other.uid) return false
if (image != other.image) return false
if (rate_id != other.rate_id) return false
return true
}
override fun hashCode(): Int {
var result = uid
result = 31 * result + image.hashCode()
result = 31 * result + rate_id.hashCode()
return result
}
}
By default when you generate the equals and hashCode (by right-clicking > "Generate..." > "equals() and hashCode()" it will look like this:
override fun equals(other: Any?): Boolean {
if (this === other) return true
if (javaClass != other?.javaClass) return false
other as Route
if (uid != other.uid) return false
if (image != other.image) return false
if (rate_id != other.rate_id) return false
if (route != other.route) return false
return true
}
override fun hashCode(): Int {
var result = uid
result = 31 * result + image.hashCode()
result = 31 * result + rate_id.hashCode()
result = 31 * result + route.hashCode()
return result
}
You have to remove the route object from both methods to stop the recursion.
IMPORTANT: You can do this on either side (Athlete or Route) to get the same result.