1. Update make a detached object persistent.
2. Config mapping relating in mapping files make query as object oriented.
3. For many-to-many relation relationship, middle table store link to two tables.
4. Key element is identify the column of middle table.
5. Db operation happend at the end of a unit work ---- session flush.
6. For value type, "element" will replace "many-to-many" in set element(acturally, for all java object, hibernate take it as a value type), value type can not shared by more than one instance.
7. All bi-directional associations need one side as inverse. bi-direction means both sides of mapping mentain relations of middle table, inverse side is a mirror of the other side, during save or update, the other side will handle.