Java.Hibernate.Medium.What is the difference between detach() and delete()?

✅ Short Answer

• detach(): removes an entity from the current persistence context — it becomes detached, but stays in the database.
• delete() (or EntityManager.remove()): marks the entity for deletion from the database, and after commit, its row is permanently removed.

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🔎 Detailed Explanation

🔹 detach()

• Detaches an entity from the first-level cache (persistence context) of the current Hibernate session or JPA EntityManager.
• After detachment:
  • The entity is still a valid Java object in your application.
  • Hibernate no longer tracks changes to it → modifications won’t be flushed to the database.
  • The database row remains unchanged.
• In JPA: entityManager.detach(entity)
  In Hibernate: session.evict(entity) (equivalent to detach).

Example:

MyEntity entity = session.get(MyEntity.class, 1L);
session.evict(entity); // Equivalent to detach
entity.setName("Won't persist!"); // Changes ignored on flush/commit

🔹 delete()

• Schedules the entity for deletion in the database — after flush() or transaction commit(), Hibernate executes a DELETE SQL statement to permanently remove the corresponding row.
• In JPA: entityManager.remove(entity)
  In Hibernate: session.delete(entity).

Example:

MyEntity entity = session.get(MyEntity.class, 1L);
session.delete(entity); // Schedules DELETE SQL → removes from DB on commit

📊 Quick Comparison Table

Feature	detach()/evict()	delete()/remove()
Effect on DB	No change → row remains	Row is permanently deleted
Effect on entity	Becomes detached, not tracked	Scheduled for deletion
Use case	Stop tracking without deleting data	Remove data entirely

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💡 Extra Insight: When to use them

✅ Use detach()/evict() when you want to avoid persisting further changes without affecting the database (e.g., memory management or skipping accidental updates).
✅ Use delete()/remove() when you want to permanently delete data from the database.

📌 Key Takeaways

✅ detach() → entity stays in the database, but Hibernate stops tracking it.
✅ delete() → entity is deleted from the database when transaction is committed.
✅ Detaching is about session/persistence context management, while deleting is about data removal.