Hibernate: merge returns a copy, and the object you passed in stays detached

merge() does not attach the instance you gave it. It copies the state onto a managed instance and returns THAT - so writes to your original after the call are silently discarded.

• Hibernate: getReference hands back a proxy that throws after the session closes
• Hibernate: without @Version, the last write silently wins
• Hibernate: orphanRemoval and CascadeType.REMOVE are different rules
• Hibernate: equals/hashCode on a generated id breaks HashSet membership
• Hibernate: the first-level cache means a re-query never sees another transaction's update
• Hibernate: flush order is fixed, so a unique constraint fails on a swap

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Published 2026-08-25

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