关联关系中，Set与List问题

[ghost]

From relational databases perspective this is a set. Databases do not preserve order and using a List is meaningless, the order in them is unspecified (unless using so called indexed collections).

Using a Set also has great performance implications. When List is used, Hibernate uses PersistentBag collection underneath which has some terrible characteristics. I.e.: if you add a new relationship it will first delete all existing ones and then insert them back + your new one. With Set it just inserts the new record.

Third thing - you cannot have multiple Lists in one entity as you will get infamous cannot simultaneously fetch multiple bags exception.

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意思是说，使用List，save的时候会先delete all，and then re-insert all，使用Set会避免该情况。

[ghost]

jeesite中对应该问题的地方是，user_role关联，saveUser的时候可以通过查看打印出的HQL语句看出。