Hibernate version: 3.2.1

Name and version of the database you are using: HSQLDB-1.8.0.7

Mapping documents:
I have three classes: Event, EventInstance and NewsItem. An Event can have multiple EventInstances, however each EventInstance belongs to one Event. Each EventInstance also belongs to one NewsItem, and one NewsItem can have multiple EventInstances. Besides that each EventInstance has two String variables. Here are the mapping files:




The problem is that this results in multiple Event objects in memory with the same ID. The SQL DB contains the proper rows, however when Hibernate loads everything to memory there are multiple Event objects being created with the same ID. I have a manager object which has a list of all Event objects loaded by Hibernate. It filled like this:


However, when I get an Event object from the EventInstance object and get the Event object with the same ID from the eventmanager Java tells me that they are different using the following code:


So two Event objects with the same ID are different. This leads to problems when, for example, removing an EventInstance for the EventInstance set of an Event object.

I really do hope somebody around here can clear this up for me, because at this point I am completely stuck with the association mapping between these objects.

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Best regards,
Jethro Borsje

Hibernate guarantees uniqueness of objects with a given id within a Session. Since your code uses a new Session for each db-access, this won't help in your case.
I'd recommend you use longer Sessions anyway, since your solution won't profit from hibernates 1st-Level-cache.

The thing to realize with hibernate is that there is only a one to one mapping between domain objects and java objects when all java objects are loaded from the same session. If you want to work with java objects loaded from different sessions then you need to think of java objects as representations of domain objects.

To grasp this consider the relationship between java objects and dates. In java, dates are represented by instances of the Date class. There is not a one to one correspondence between dates and instances of Date. For example to represent the date January 1, 1970, 00:00:00 GMT in java you can use the following code:



This creates a representation of the date January 1, 1970, 00:00:00 GMT. Now if you execute the following code:



it will print out false. Why? - because, even though date1 and date2 are identical dates, each points to a different representation of the same date.

Hopefully the example with Dates is obvious. So how does this relate to your problem?

In the code you submitted:



instanceEvent and managerEvent each points to different representations of the same domain object. This happens because you loaded these objects in different hibernate sessions.

If you want to work with objects loaded in different hibernate sessions then you must implement equals and hashCode correctly. See http://www.hibernate.org/hib_docs/v3/reference/en/html_single/#persistent-classes-equalshashcode for details. This should hopefully solve the problems you are getting with removing objects from a Set.

I also recommend reading http://www.hibernate.org/109.html[/code]

I am not really sure how I use a new session every time. By using the "getCurrentSession()" method I do not necessarily create a new session, do I?

I understand the example, it exactly demonstrates what I do not want. I want to have the same Java object for the same database row.


This is what I do not really understand. How / where did I create a new sessions?


I have implemented the equals() and hashcode() methods and as far as I can see they work properly. The following code:

Results in this:


So the two different EventInstance objects have the same hashcode and are equal!

--
Best regards,
Jethro Borsje

Hi everybody,

I played around with it a bit more and figured out the problem is that somehow Hibernate does not properly uses my hashcode() and equals() methods. If I use those methods Java tells me my objects are the same, yet Hibernate creates them separately instead of returning the already created object.

--
Best regards,
Jethro Borsje

Hi everybody,

I played around with it a bit more and figured out the problem is that somehow Hibernate does not properly uses my hashcode() and equals() methods. If I use those methods Java tells me my objects are the same, yet Hibernate creates them separately instead of returning the already created object.

--
Best regards,
Jethro Borsje

I am sorry for double posting, is there a way to remove a post?

After more checking I found the problem of duplicate objects is happening everywhere in my application, not just in the aforementioned objects. This happens despite having implemented correct equals() and hashCode() methods everywhere.

Can this have something to do with the fact that I am not using a cache? This what my configuration looks like:


Or is there some other setting that I need to set to let hibernate use the equals() and hashCode() methods properly?

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Best regards,
Jethro Borsje

http://www.hibernate.org/hib_docs/v3/reference/en/html_single/#tutorial-firstapp-workingpersistence says:

Hi Jethro

You said:



Implementing equals() and hashCode() will not rid your application of duplicate objects. All implementing equals() and hashCode() will do is enable you to work with objects loaded from different hibernate sessions.

You said on Wed Jul 25, 2007 at 12:41 pm:



It is difficult to make my point clear without knowing how you create the hibernate sessions. Instead please consider the following code:



The above code will print out "duplicate java objects created". Of course the example needs some other supporting code but I hope it serves to help explain an important point. domain objects loaded in different sessions in this way will result in duplicate java objects.

You also said on Wed Jul 25, 2007 at 12:41 pm:



I think there are only two ways of avoiding duplicate objects.

1. Make sure your application never uses objects loaded from different sessions together

2. To re-attach the objects you want to use to the new session before loading other objects from the session by using Session.update() or Session.lock().

I word of warning. I have personally found avoiding duplicate objects very difficult to do.

Allright, that's clear. Thank you for clearing it up.



Here is my code which loads objects:

Both the events and the newsitems are loaded this way.


Allright, now I am a bid confused, because if "eventFromFirstSession == eventFromSecondSession" is true, doesn't this mean that the two objects are exactly the same (the same in memory), so this means that they are not duplicate...


Ad 1: in my previous code example: how did I close the session? E.g., how can I load everything in one session, doesn't getCurrentSession() return the same sessions each time?

Ad 2: If I chose this approach I should either use session.update() or session.lock() to add my already loaded objects to the new session?

Thank you very much for all you help, I think we are close to finding a solution.

--
Best regards,
Jethro

Putting all the load code between

and

Seems to do the trick.

Thanks for the help!