Hibernate throws MultipleBagFetchException - cannot simultaneously fetch multiple bags

Question

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 lt Child gt  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 lt Child gt  children       AnotherParent java   Entity public AnotherParent      Id   GeneratedValue strategy GenerationType IDENTITY   private Long id     OneToMany mappedBy  parent   fetch FetchType EAGER   private List lt AnotherChild gt  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

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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 lt Model gt  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

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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

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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

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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

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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

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I solved by annotating   OneToMany cascade   CascadeType ALL  fetch   FetchType LAZY

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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 lt Child gt  children           in Children then you should add the orderIndex field

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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 lt Child gt  is more appropriate than List lt Child gt   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

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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

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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 lt Post gt  posts   entityManager  createQuery       quot select p  quot         quot from Post p  quot         quot left join fetch p comments  quot         quot left join fetch p tags  quot         quot where p id between  minId and  maxId quot   Post class   setParameter  quot minId quot   1L   setParameter  quot maxId quot   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  quot solution quot  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  quot fix quot   The proper solution You can do the following trick  List lt Post gt  posts   entityManager  createQuery       quot select distinct p  quot         quot from Post p  quot         quot left join fetch p comments  quot         quot where p id between  minId and  maxId  quot   Post class   setParameter  quot minId quot   1L   setParameter  quot maxId quot   50L   setHint QueryHints PASS DISTINCT THROUGH  false   getResultList     posts   entityManager  createQuery       quot select distinct p  quot         quot from Post p  quot         quot left join fetch p tags t  quot         quot where p in  posts  quot   Post class   setParameter  quot posts quot   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

User · Answer

Add a Hibernate-specific  Fetch annotation to your code    OneToMany mappedBy  parent   fetch FetchType EAGER   Fetch value   FetchMode SUBSELECT  private List lt Child gt  childs    This should fix the issue  related to Hibernate bug HHH-1718

User · Answer

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

User · Answer

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

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To fix it simply take Set in place of List for your nested object    OneToMany Set lt Your object gt  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

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Commenting both Fetch and LazyCollection sometimes helps to run project    Fetch FetchMode JOIN   LazyCollection LazyCollectionOption FALSE

User · Answer

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

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

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