I'm experiencing an unique constraint violation when updating a many-to-many association if the hashCode() method of the entities is based on fields which have a different value than in the database.

What I try to do in two subsequent web requests (that means the session is closed in-between):

Store two objects with the following data:

Code:
Parent:
  idField1   = 1
  dataField1 = dataParent
  systemId   = 3
  Set<Child> = {child} (many-to-many association)




Code:
Child:
  idField2   = 1
  dataField2 = dataChild
  systemId   = 3



The tables PARENT, CHILD and PARENTS2CHILD are populated correctly.

Now I try to merge and flush the previously saved objects with the following data:

Code:
Parent:
  idField1   = 1
  dataField1 = dataParent
  systemId   = 4                 //this value has changed
  Set<Child> = {child} (many-to-many association)




Code:
Child:
  idField2   = 1
  dataField2 = dataChild
  systemId   = 4                 //this value has changed



The tables PARENT, CHILD are updated correctly. But Hibernate tries to insert a new row in the relation table PARENTS2CHILD and an Unique Constraint Violation occurs.

The reason is that the systemId is part of the hashCode generation for the Parent and Child classes (see below). This behavior is generated by AndroMDA out of our UML model automatically. So the entities are first added to a Set internally by Hibernate after the initial load from the database (with systemId=3) and during the check for necessary updates the entities with systemId=4 are considered to be missing (since they can not be found in the internal Set because their hashCode has changed).

I know that this behavior violates the contract of the Set specification, but from the view of a Hibernate user one can not know that the implementation of a by value equality/hashCode (as mentioned in the book Java Persistence with Hibernate) leads to the described problem. In this book, the by value equality/hashCode is not recommended, but also not completely forbidden, but when using many-to-many associations it seems to be problematic.

In http://www.hibernate.org/109.html and in other discussions I found very few comments on by value equality/hashCode so I think it is not used widely, am I right?

In our case, I think we will change the generation process for our Entity classes in a way that the equals and hashCode methods only depend on the database identifier, but I would be interested in your opinion about using by value equality/hashCode generated by default for all Entities in a class hierarchy which have no specified identifier(like in this case GeneralEntity, see below).

Cheers,
Jürgen


Hibernate version:
Experienced with 3.2.4.sp1 and 3.2.6.GA

Mapping documents:

GeneralEntity:


Parent extends GeneralEntity:


Child extends GeneralEntity:


Code between sessionFactory.openSession() and session.close():


Full stack trace of any exception that occurs:


Name and version of the database you are using:

Oracle 10.1.02

The generated SQL (show_sql=true):


Debug level Hibernate log excerpt:

Have you checked your unique constraint on the database?
It has to be defined on both columns.