[java] Hibernate throws MultipleBagFetchException - cannot simultaneously fetch multiple bags

Hibernate throws this exception during SessionFactory creation:

org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags

This is my test case:

Parent.java

@Entity
public Parent {

 @Id
 @GeneratedValue(strategy=GenerationType.IDENTITY)
 private Long id;

 @OneToMany(mappedBy="parent", fetch=FetchType.EAGER)
 // @IndexColumn(name="INDEX_COL") if I had this the problem solve but I retrieve more children than I have, one child is null.
 private List<Child> children;

}

Child.java

@Entity
public Child {

 @Id
 @GeneratedValue(strategy=GenerationType.IDENTITY)
 private Long id;

 @ManyToOne
 private Parent parent;

}

How about this problem? What can I do?

EDIT

OK, the problem I have is that another "parent" entity is inside my parent, my real behavior is this:

Parent.java

@Entity
public Parent {

 @Id
 @GeneratedValue(strategy=GenerationType.IDENTITY)
 private Long id;

 @ManyToOne
 private AnotherParent anotherParent;

 @OneToMany(mappedBy="parent", fetch=FetchType.EAGER)
 private List<Child> children;

}

AnotherParent.java

@Entity
public AnotherParent {

 @Id
 @GeneratedValue(strategy=GenerationType.IDENTITY)
 private Long id;

 @OneToMany(mappedBy="parent", fetch=FetchType.EAGER)
 private List<AnotherChild> anotherChildren;

}

Hibernate doesn't like two collections with FetchType.EAGER, but this seems to be a bug, I'm not doing unusual things...

Removing FetchType.EAGER from Parent or AnotherParent solves the problem, but I need it, so real solution is to use @LazyCollection(LazyCollectionOption.FALSE) instead of FetchType (thanks to Bozho for the solution).

you can keep booth EAGER lists in JPA and add to at least one of them the JPA annotation @OrderColumn (with obviously the name of a field to be ordered). No need of specific hibernate annotations. But keep in mind it could create empty elements in the list if the chosen field does not have values starting from 0

 [...]
 @OneToMany(mappedBy="parent", fetch=FetchType.EAGER)
 @OrderColumn(name="orderIndex")
 private List<Child> children;
 [...]

in Children then you should add the orderIndex field

We tried Set instead of List and it is a nightmare: when you add two new objects, equals() and hashCode() fail to distinguish both of them ! Because they don't have any id.

typical tools like Eclipse generate that kind of code from Database tables:

@Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((id == null) ? 0 : id.hashCode());
    return result;
}

You may also read this article that explains properly how messed up JPA/Hibernate is. After reading this, I think this is the last time I use any ORM in my life.

I've also encounter Domain Driven Design guys that basically say ORM are a terrible thing.

Commenting both Fetch and LazyCollection sometimes helps to run project.

@Fetch(FetchMode.JOIN)
@LazyCollection(LazyCollectionOption.FALSE)

You could use a new annotation to solve this:

@XXXToXXX(targetEntity = XXXX.class, fetch = FetchType.LAZY)

In fact, fetch's default value is FetchType.LAZY too.

When you have too complex objects with saveral collection could not be good idea to have all of them with EAGER fetchType, better use LAZY and when you really need to load the collections use: Hibernate.initialize(parent.child) to fetch the data.

For me, the problem was having nested EAGER fetches.

One solution is to set the nested fields to LAZY and use Hibernate.initialize() to load the nested field(s):

x = session.get(ClassName.class, id);
Hibernate.initialize(x.getNestedField());

To fix it simply take Set in place of List for your nested object.

@OneToMany
Set<Your_object> objectList;

and don't forget to use fetch=FetchType.EAGER

it will work.

There is one more concept CollectionId in Hibernate if you want to stick with list only.

But remind that you won't eliminate the underlaying Cartesian Product as described by Vlad Mihalcea in his answer!

Add a Hibernate-specific @Fetch annotation to your code:

@OneToMany(mappedBy="parent", fetch=FetchType.EAGER)
@Fetch(value = FetchMode.SUBSELECT)
private List<Child> childs;

This should fix the issue, related to Hibernate bug HHH-1718

At my end, this happened when I had multiple collections with FetchType.EAGER, like this:

@ManyToMany(fetch = FetchType.EAGER, targetEntity = className.class)
@JoinColumn(name = "myClass_id")
@JsonView(SerializationView.Summary.class)
private Collection<Model> ModelObjects;

Additionally, the collections were joining on the same column.

To solve this issue, I changed one of the collections to FetchType.LAZY since it was okay for my use-case.

Goodluck! ~J

One good thing about @LazyCollection(LazyCollectionOption.FALSE) is that several fields with this annotation can coexist while FetchType.EAGER cannot, even in the situations where such coexistence is legit.

For example, an Order may have a list of OrderGroup(a short one) as well as a list of Promotions(also short). @LazyCollection(LazyCollectionOption.FALSE) can be used on both without causing LazyInitializationException neither MultipleBagFetchException.

In my case @Fetch did solve my problem of MultipleBacFetchException but then causes LazyInitializationException, the infamous no Session error.

Considering we have the following entities:

And, you want to fetch some parent Post entities along with all the comments and tags collections.

If you are using more than one JOIN FETCH directives:

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();

Hibernate will throw the infamous:

org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags [
  com.vladmihalcea.book.hpjp.hibernate.fetching.Post.comments,
  com.vladmihalcea.book.hpjp.hibernate.fetching.Post.tags
]

Hibernate doesn't allow fetching more than one bag because that would generate a Cartesian product.

The worst "solution"

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. Don't do that!

Using Sets instead of Lists will make the MultipleBagFetchException go away, but the Cartesian Product will still be there, which is actually even worse, as you'll find out the performance issue long after you applied this "fix".

The proper solution

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();

In the first JPQL query, distinct DOES NOT go to the SQL statement. That's why we set the PASS_DISTINCT_THROUGH JPA query hint to false.

DISTINCT has two meanings in JPQL, and here, we need it to deduplicate the Java object references returned by getResultList on the Java side, not the SQL side.

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.

There's more you could do

If you're using the FetchType.EAGER strategy at mapping time for @OneToMany or @ManyToMany associations, then you could easily end up with a MultipleBagFetchException.

You are better off switching from FetchType.EAGER to Fetchype.LAZY since eager fetching is a terrible idea that can lead to critical application performance issues.

Conclusion

Avoid FetchType.EAGER and don't switch from List to Set just because doing so will make Hibernate hide the MultipleBagFetchException under the carpet. Fetch just one collection at a time, and you'll be fine.

As long as you do it with the same number of queries as you have collections to initialize, you are fine. Just don't initialize the collections in a loop, as that will trigger N+1 query issues, which are also bad for performance.

I found a good Blog post about the behaviour of Hibernate in this kind of object mappings: http://blog.eyallupu.com/2010/06/hibernate-exception-simultaneously.html

Simply change from List type to Set type.

But remind that you won't eliminate the underlaying Cartesian Product as described by Vlad Mihalcea in his answer!

I solved by annotating:

@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)

After trying with every single option describe in this posts and others, I came to the conclusion that the the fix is a follows.

In every XToMany place @XXXToMany(mappedBy="parent", fetch=FetchType.EAGER) and intermediately after

@Fetch(value = FetchMode.SUBSELECT)

This worked for me