I've read many threads into this forum talking about parent/child association but I didn't find any real answer about the following mapping issue :

Here is the association I want to map :



which means :

* a Task may have 0,n sub-tasks (but a sub-task is a Task)
* a subTask has always access to its parent Task.

Based on what I've read, this is pretty easy if the cardinality between a subTask and its parent Task is (0,1). In this case, a foreign key "parentTask" can easily be added to Task class to get a fully bidirectionnal one-to-many association.

However, in my case, I want from a subtask to be able to get the parent Task (cardinality 1,1) referenced into the entity table ....

but there's no way to specify a table name into a "many-to-one or a one-to-one" association.

Is my approach completely wrong ?
Any clue ?

What you need is a many-to-one not-null="true" and a DB having a not null constraint on it.
But what about RootTask ?

What is a RootTask for u ? coming back to your comment, the many-to-one association with not-null="true" can only (tell me if i'm wrong) be done a column of the Task class.

What I would like (but may be you have a better approach) is to map from the Task class a one-to-many association to the "association" entity class between Task and Task. Is it more clear or am I confusing you ?

Ok ... sorry ... I've just understand your question : "but what about the RootTask ?" ... and this is exactly my problem :

* there's a one-to-many association from Task to itself.
* The RootTask has a one-to-many assocation to itself, the cardinality being (0,n) AND that's the issue !
* It means that the RootTask can't have any parent (or maybe itself)
* a sub-task has a mandatory parent Task (1,1 cardinality)

I think we have the same understanding of the problem.

Well, I don't have a magic solution ;-(
I've never try to reference a "root" row having a not null constaint and a FK to itself. I don't think the DB allows that.

A better approch may be to add some Lifecycle or some interceptor in hibernate to check the cardinality and manage the root case.

Any other ideas ?

Thanks Emmanuel for your ideas ; managing it via an Interceptor or a Lifecycle is clever ; I will try a proof-of-concept in this way.

My only concern is that I thought that this kind of mapping is a common one and that Hibernate offers a simple mapping solution for it but that's not the case.

In fact, Object mapping with Hibernate for a newbie is not so trivial and I will say that the only concern with Hibernate documentation I have is that it's really dedicated to users already knowing Object mapping concepts ... :-((

Anyway, thanks for you help, any other ideas guys ?

When I started with Hibernate, I was kind of a rookie on O/R mapping. After reading the doc several times and analysing every single word of it, I learnt a lot.
Using this kind of tools wo knowledge is like using JDBC wo knowing SQL, you make architecture and design issues.
This is a vicious circle, but it can become a virtuous one with work.