I have a really simple problem where I want to create hierarchical "Category" where I'm using a legacy mysql database structure. (I searched the forums for a similar problem but couldn't find anything)

The current structure defines a Category as being a root Category if the parent_category column value is 0.

So in my hibernate mapping I have the following:




Is there a way to define the many-to-one mapping such that it the parent object will be null? I keep getting a "UnresolvableObjectException: No row with the given identifier exists: 0"

I would rather not redefine my mysql database because it's a legacy structure and I am still planning on using the older software that accesses it.

This is an application level business rule which is not relational - you need to manage the association from that layer not at the mapping level, eg, an appropiate DAO will do a load on the from the property value.

Alternatively, you could code up a user type but i'm not sure it worth the effort.

I'm not sure that actually solves my problem... or at least I'm not following your statement.

Under the assumption that the "Category" is a class where "parent" is a member variable of type "Category" and also following the assumption that the legacy database is using "0" to mean "NULL" mean in that a category with a "0" parent is a root object.

Is there any way that I can create a mapping that

I'm expecting a "You can't. You have to modify your database structure or explicitly capture the parent ID and then query the DB with the ID appropropiate ID value".

though I'm hoping there might be a means of mapping a value as a null association.

Your expectation is correct unless you code a custom user type. So it can be done but you have to put some work into it.

I now know what you mean my UserType. I'll have to look into it. (Apparently it's used for enumerations as well).

Jeremy

Correct. It is actually a very powerfull aspect once you comfortable with the plumbing involved.

hi,

for using '0' as 'null' you can take a look at this thread: http://forum.hibernate.org/viewtopic.php?t=935678&highlight=

There you can even find an example-UserType.

gtx
curio

Hi curio & co

Thanks a lot for posting your thoughts in thread: http://forum.hibernate.org/viewtopic.php?t=935678I asked the very same thing already twice in the forum without getting a usefull answer.

I've also a similar but more complicated problem. In order no to capture this thread I opened a new one at http://forum.hibernate.org/viewtopic.php?t=936260

Any ideas are very welcome.
TIA
Ernst