org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags
bag, hibernate, hibernate-mapping, java, jpa
Solution
Hibernate doesn't allow fetching more than one bag because that would generate a Cartesian product.
Now, you will find lots of answers, blog posts, videos, or other resources telling you to use a `Set` instead of a `List` for your collections.
That's terrible advice!
Using `Sets` instead of `Lists` will make the `MultipleBagFetchException` go away, but the Cartesian Product will still be there.
The right fix
Instead of using multiple `JOIN FETCH` in a single JPQL or Criteria API query:
List<Post> posts = entityManager.createQuery("""
select p
from Post p
left join fetch p.comments
left join fetch p.tags
where p.id between :minId and :maxId
""", Post.class)
.setParameter("minId", 1L)
.setParameter("maxId", 50L)
.getResultList();
You can do the following trick:
List<Post> posts = entityManager.createQuery("""
select distinct p
from Post p
left join fetch p.comments
where p.id between :minId and :maxId
""", Post.class)
.setParameter("minId", 1L)
.setParameter("maxId", 50L)
.setHint(QueryHints.PASS_DISTINCT_THROUGH, false)
.getResultList();
posts = entityManager.createQuery("""
select distinct p
from Post p
left join fetch p.tags t
where p in :posts
""", Post.class)
.setParameter("posts", posts)
.setHint(QueryHints.PASS_DISTINCT_THROUGH, false)
.getResultList();
As long as you fetch at most one collection using `JOIN FETCH`, you will be fine. By using multiple queries, you will avoid the Cartesian Product since any other collection but the first one is fetched using a secondary query.
Problem
Following is my code Here I am using multiple lists to fetch data from database. On fetching data from hql query it is showing exception. Pojo Class ``` public class BillDetails implements java.io.Serializable { private Long billNo; // other fields @LazyCollection(LazyCollectionOption.FALSE) private List<BillPaidDetails> billPaidDetailses = new ArrayList<BillPaidDetails>(); private Set productReplacements = new HashSet(0); @LazyCollection(LazyCollectionOption.FALSE) private List<BillProduct> billProductList = new ArrayList<BillProduct>(); //getter and setter } ``` hmb.xml file ``` <class name="iland.hbm.BillDetails" table="bill_details" catalog="retail_shop"> <id name="billNo" type="java.lang.Long"> <column name="bill_no" /> <generator class="identity" /> </id> <bag name="billProductList" table="bill_product" inverse="true" lazy="false" fetch="join"> <key> <column name="bill_no" not-null="true" /> </key> <one-to-many class="iland.hbm.BillProduct" /> </bag> <bag name="billPaidDetailses" table="bill_paid_details" inverse="true" lazy="false" fetch="select"> <key> <column name="bill_no" not-null="true" /> </key> <one-to-many class="iland.hbm.BillPaidDetails" /> </bag> <set name="productReplacements" table="product_replacement" inverse="true" lazy="false" fetch="join"> <key> <column name="bill_no" not-null="true" /> </key> <one-to-many class="iland.hbm.ProductReplacement" /> </set> </class> ``` Hql query ``` String hql = "select distinct bd,sum(bpds.amount) from BillDetails as bd " + "left join fetch bd.customerDetails as cd " + "left join fetch bd.billProductList as bpd " + "left join fetch bpd.product as pd " +"left join fetch bd.billPaidDetailses as bpds " + "where bd.billNo=:id " + "and bd.client.id=:cid "; ``` I am trying following query to fetch data from database but this is showing `org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags` How to resolve this