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.