@OneToMany/ManyToOne and @OneToOne relationship between same entities.

[Adriano Meis]

I explain quickly what I should do:

I've trees, every tree has many branches. That's easy.

in pseudo-code:

@Entity Tree
-> @Id Integer treeId;
-> @OneToMany List<Branch> branches;
-> String treeName;

PK: [treeId]

@Entity Branch
-> @Id @ManyToOne Tree tree
-> @Id Integer branchId
-> String branchName;

PK: [tree, branchId]
(of course i used an embeddedId for the entity Branch.)

Well, what I wrote, works.
Now.. my problem:
I want to be able to know which is the last grown branch of a specific tree.

From an Entity-Relation point of view, I want to add a @OneToOne relationship between Tree and Branch: a Tree will have just ONE lastBranch.

So the Tree entity should become something like this:

@Entity Tree
-> @Id Integer treeId;
-> @OneToMany List<Branch> branches;
-> String treeName;
-> @OneToOne Branch lastBranch;

But i'm not very sure about this:

1) a relationship to a Branch should be mapped on the DB schema with two columns: the Branch composite id [TREE_ID, BRANCH_ID], in the case of Tree entity, it already have the TREE_ID value (since it is its primary key), so it will be so clever to not duplicate the TREE_ID column?

2) How should I map the inverse @OneToOne relationship "lastBranch" from Branch to Tree? Should not map it because it's already mapped by the @ManyToOne tree?

PS:
Surely some of you would suggest to dinamically extract in someway the last branch from the branches list, using some "age" attribute, sorting it, and getting last element.. but I want a more performant solution, and a solution i can use easily from HQL and OGNL, something like this:

mytree.lastBranch.branchName

I think that the @Formula solution can have some performance issues.. so i would like to phisically add this @OneToOne relationship on the tables..

Thank you

[Ray Johnson]

can think a simple solution to the problem
- use order by in braches by date (in hbm file) and pick the last one from collection wherever needed.

otherwise, write a getLastBranch in Tree class which loops through your branches to find the last branch and return. - These is a very light weight solution.

afik, for one to one you must find a unique identifier in the child table as well that differentiates the unique rows involved (in your case last created branches). I hope date field or id field might not help in your case. Hope this helps.