# MultipleBagFetchException: cannot simultaneously fetch multiple bags

[TOC]

###### tags: `hibernate`

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> Fetching two or more Bags at the same time on an Entity could form a Cartesian Product. Since a Bag doesn't have an order, Hibernate would not be able to map the right columns to the right entities. Hence, in this case, it throws a MultipleBagFetchException.

## 解決方案一

在 entity class 的`Collection`attribute 加上

@LazyCollection(LazyCollectionOption.FALSE)

若原本有

@ManyToMany(fetch = FetchType.*)

則另須將`fetch = FetchType.*`拿掉。

出處：[Hibernate throws MultipleBagFetchException - cannot simultaneously fetch multiple bags](https://stackoverflow.com/questions/4334970/hibernate-throws-multiplebagfetchexception-cannot-simultaneously-fetch-multipl)

## 解決方案二

* 將`Collection`改為`Set`。
* 將`Collection`改為`List`。
* 使用 multiple queries (建議)。

出處：[A Guide to MultipleBagFetchException in Hibernate](https://www.baeldung.com/java-hibernate-multiplebagfetchexception)