JPA Entity Mapping for Kotlin

Kotlin's data class is natural for DTOs but dangerous for JPA entities. Hibernate relies on identity semantics that data class breaks: equals/hashCode over all fields corrupts Set/Map membership after state changes, and auto-generated copy() creates detached duplicates of managed entities.

This skill teaches correct entity design, identity strategies, and uniqueness constraints for Kotlin + Spring Data JPA projects.

Entity Design Rules

Never use data class for JPA entities. Use a regular class. Keep data class for DTOs.
Keep transport DTOs and persistence entities separate unless the project clearly uses a shared model.
Model required columns as non-null only when object construction and persistence lifecycle make it safe.
Use lateinit only when the project already accepts that tradeoff and the lifecycle is safe.
Verify kotlin("plugin.jpa") or equivalent no-arg support when JPA entities exist.
Verify classes and members are compatible with proxying where needed.

Identity and Equality

Never accept all-field equals/hashCode generated by data class on an entity.
Follow project conventions when they already define an identity strategy.
If no convention exists, use ID-based equality with a stable hashCode.
Be explicit about mutable fields and lazy associations when discussing equality.

Broken: `data class` Entity

// WRONG: data class generates equals/hashCode from ALL fields
data class Order(
    @Id @GeneratedValue val id: Long = 0,
    var status: String,
    var total: BigDecimal
)
// BUG: order.status = "SHIPPED"; set.contains(order) → false (hash changed)
// BUG: Hibernate proxy.equals(entity) → false (proxy has lazy fields uninitialized)

Correct: Regular Class with ID-Based Identity

@Entity
@Table(name = "orders")
class Order(
    @Column(nullable = false)
    var status: String,

    @Column(nullable = false)
    var total: BigDecimal
) {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    val id: Long = 0

    override fun equals(other: Any?): Boolean {
        if (this === other) return true
        if (other !is Order) return false
        return id != 0L && id == other.id
    }

    override fun hashCode(): Int = javaClass.hashCode()

    // toString must NOT reference lazy collections
    override fun toString(): String = "Order(id=$id, status=$status)"
}

Key rules:

equals compares by ID only — stable under dirty tracking and proxy unwrapping
hashCode returns class-based constant — avoids Set/Map corruption after persist
toString excludes lazy-loaded relations — prevents LazyInitializationException
Constructor params are mutable entity fields; id is val with default

Uniqueness Constraints

When an API must be idempotent (e.g., "reserve stock for order X"), enforce uniqueness at both layers: database constraint for correctness, application check for clean errors.

Broken: No Duplicate Guard

@Service
class ReservationService(private val repo: ReservationRepository) {
    @Transactional
    fun createReservation(variantId: Long, orderId: String, qty: Int): Reservation {
        // BUG: no check — duplicates silently accumulate
        return repo.save(Reservation(variantId = variantId, orderId = orderId, quantity = qty))
    }
}

Correct: Database Constraint + Application Guard

@Entity
@Table(
    name = "reservations",
    uniqueConstraints = [
        UniqueConstraint(columnNames = ["variant_id", "order_id"])
    ]
)
class Reservation(
    @Column(name = "variant_id", nullable = false)
    val variantId: Long,

    @Column(name = "order_id", nullable = false)
    val orderId: String,

    @Column(nullable = false)
    var quantity: Int
) {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    val id: Long = 0
}

interface ReservationRepository : JpaRepository<Reservation, Long> {
    fun findByVariantIdAndOrderId(variantId: Long, orderId: String): Reservation?
}

@Service
class ReservationService(private val repo: ReservationRepository) {
    @Transactional
    fun createReservation(variantId: Long, orderId: String, qty: Int): Reservation {
        repo.findByVariantIdAndOrderId(variantId, orderId)?.let {
            throw IllegalStateException(
                "Reservation already exists for variant=$variantId, order=$orderId"
            )
        }
        return repo.save(Reservation(variantId = variantId, orderId = orderId, quantity = qty))
    }
}