What I am doing that I am worried is the wrong thing to do is read an object from the database clear out any lists on that object and then re-populate them - potentially with the same data.

I am using NHibernate 1.2.0 in the persistence layer of a multi tier system. The data from the database is being passed upto a front end via a web service.

The top level object I have is Contact which has a number of one to many relationships hanging of it, such as ContactAddress and ContactLanguage. The mapping for Contact looks something like this:

When I receive a contact from the front-end for writing back to the database, I need to translate the "front-end" contact into a "NHibernate" contact. To do this I re-read the contact from the database, I then use the "front-end" contact to set the new values in the "NHibernate" Contact. For the properties such as forename and surname, this is a straight forward task:

The lists however is where I am woried that what I am doing is fundamentally wrong.

Taking the ContactAddressList as an example, the first thing I do is clear out the list on the "NHibernate" contact. I will then loop through the list of addresses on the front-end contact and for each one, attempt to read it from the database. If the address exists, I add it back into the address list on the "NHibernate" contact, if it doesn't I create a new "NHibernate" address and add that into the list. Finally I update any of the properties on the "NHibernate" contact address with what came back from the front-end. Any addresses that were deleted by the front-end, won't be added into the list and since I have cascade set to all-delete-orphan, they will be deleted, the new ones will be added and any others will be updated:


I would really appreciate it if someone could confirm one way or another whether what I am doing is OK or not.

It has been suggested to me that the reason I haven't got any responses to my question is that I take a bit too long to get to my point :o). Please find the point at the begining of my post now:



A quick yes or no answer would be really appreciated.

Thanks.

I am going to take it upon myself to say that yes this wrong!

The reason I asked the question in the first place was because although this approach seemed to work fine for me for some time, I recently added another one to many relationship to the contact and I started getting an ObjectDeletedException when I was processing this new relationship. Specifically, after I had cleared out the list, when I tried reading (what I considered to be) an existing item, the exception was thrown.

I spent time trying to debug the NHibernate code in addition to my own in the hope that if nothing else I could see when and where NHibernate was trying to delete my objects. I never did track it down but I finally decided to add a flush just before I did anything with the NHibernate session (whether that be a read a save or whatever). This resulted in ObjectDeletedExceptions being thrown whenever I tried reading an object that was previously in a list that I had cleared out. I could at this point try and argue that this is a bug - I won't I'll just post my soluition instead.

Basically what I do now is loop through the list that I received from the client and attempt to read each corresponding object from the database. If one is found then I update it with the data received from the client. When I do my read, NHibernate will return me a reference to the same object that is in the list that I was previously clearing out (I know this isn't strictly true due to lazy loading, but is essentially what is happening) - therefore I am effectively updating both lists at this point. If I fail to read an item from the database I create a new object instead and add that to my list. Once I have processed the list received from the client I am left with two lists. The one NHibernate created which I was previously clearing out and the one I just created. The first one won't have any new items in it, so I need to get these from the second list and add them to the first. Any items that are in the first list but don't appear in the second will need to be removed from the first list. The code I use to do this is as follows:


The static DataService methods are as follows:


I hope this means that somebody else won't spend two days of there life on the same issue :o)

I don't know if this is the issue that you ran into or not, but, one thing to watch out for is that when you do a query, I'm pretty sure by default it will flush any changes that you have made to objects that are in the session cache. You can change the flush mode to change how this works. I'm not saying that you should change the flush mode, just that it can be done and pointing out that the session gets flushed on a query.