العودة إلى حالات الكيان: merge مقابل persist

في هذا الدرس

handle detached entities correctly.

الموضوعات المخطط لها

• States: transient (new), managed (in the persistence context), detached (loaded earlier, session closed), removed.
• persist makes a new entity managed; merge copies the state of a detached entity onto a managed copy and returns that copy.
• Use the returned object from merge; the argument stays detached.
• Spring Data save calls persist for new entities and merge otherwise; merge of an unknown id may do an extra SELECT.
• Prefer loading the entity inside the transaction and changing it (dirty checking) over merging DTO-built entities.