What do REFRESH and MERGE mean in terms of databases?
JPA Annotation Meaning for Many to Many relationships:
- ALL - all possible cascading operations performed on the source entity are cascaded to the target of the association.
- MERGE - if the source entity is merged, the merge is cascaded to the target of the association.
- PERSIST - if the source entity is persisted, the persist is cascaded to the target of the association.
- REFRESH - if the source entity is refreshed, the refresh is cascaded to the target of the association.
- REMOVE - if the source entity is removed, the target of the association is also removed.
I myself see them this way (more readable):
- MERGE - for existing objects, to merge the existing data in the table with the data in my object. (sync to database)
- PERSIST - is create new records from object in the database.
- REFRESH - is to refresh the data in the object. Perhaps there was a change on the database which needs to be synced. (sync from database)
- REMOVE - is, well, delete.
REFRESH means "pull any state changes from the database into my representation". Cascading this is simple; it means that all associated entities are refreshed.
MERGE means something complex that approximates "save" but is more like "push this detached entity back into managed status and save its state changes"; the cascading means that all associated entities get pushed back the same way, and the managed-entity handle you get back from .merge() has all managed entities associated with it.