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).
I think a newer version of hibernate (supporting JPA 2.0) should handle this. But otherwise you can work it around by annotating the collection fields with:
@LazyCollection(LazyCollectionOption.FALSE)
Remember to remove the fetchType
attribute from the @*ToMany
annotation.
But note that in most cases a Set<Child>
is more appropriate than List<Child>
, so unless you really need a List
- go for Set
But remind that with using sets you won't eliminate the underlaying Cartesian Product as described by Vlad Mihalcea in his answer!