I have three classes A, B, C as shown below.





I have mapped A and C using hibernate to tables T_A and T_C.

My question is, what mapping should I use for class B? Since its only a container for instances of C in an array and does not have any other attributes, it cannot have its own table. Yet hibernate has to know that it needs to create instance of B, and add instances of "C" to it.

Appreciate any responses.

You should map B as a "component" class of A. Example from the docs:


Person has a table but Name doesn't. The fields in Name are populated by entries from the person table.

Similarly, the fields of B could be populated by whatever database linkage there is between A and C.

Thanks. I looked at "component" tag, but it doesn't support <one-to-many> mapping from B to C:


Are there any workarounds, should "arrays" be used instead?

You need to use the array/list/set tag like you would for any other one-to-may:

Two more questions. Thank you for the answers.

1)


is "a_id" in this case the foreign key in "TABLE_C" from the "TABLE_A"? Does "key column" denote a foreign key in "TABLE-A" or "TABLE-C".

2) I read relevant parts of the hibernate manual, but have some confusion about where foreign keys need to be created, and how the mapping of keys is defined.

The following mapping for the case below does not work. The problem with this seems to be that it expects a foreign key "e_id" from "TABLE_E" to be in TABLE_D. But that is reversing the parent-child relationship.

2.1) Should the mapping instead be defined as <many-to-one> in E.hbm.xml, and omit the <one-to-many> in D.hbm.xml? Then E cannot be navigated from D.

2.2) When defining <one-to-many> in D.hbm.xml, the "TABLE_E.d_id" foreign key is not "accessible", correct?

Have you read the following two chapters in the Hibernate reference documentation? If not, I think you should since it explains a lot of things, with <many-to-one>, <one-to-many> and more...

http://www.hibernate.org/hib_docs/v3/re ... tions.html

http://www.hibernate.org/hib_docs/v3/re ... bidir.html

Yes, a_id is a column in table C that refers to the primary key of A, as a foreign key. This is how you know which instances of C belong to each A.

2 - Its unclear why a foreign key to E is expected to be in D. Perhaps your mapping of the reverse relationship from E to D (not shown) is incorrect.

2.1 - If you need to navigate in both directions you can define the mapping as one-to-many in the parent AND many-to-one in the child to create a bi-directional relationship. Remember to set the inverse="true" flag on one end - probably the parent.

2.2 - Well, if you define a uni-directional relationship from D to E then run a query that returns you instances of E then you're correct, you can't access the column value even though its there in the db. But if you need D from E you wouldn't do this.

If you navigate to E from an instance of D you already know the value of the column in E - its the primary key of that instance of D.

If you have a bi-directional relationship and you run the query for E then you also know the value from the primary key of the associated D.

Are the mappings below correct?



But this doesn't populate the FK E.d_id.

What I want to do is:
1) Navigate D->E
2) Use FK d_id in E to denote parent-child relationship D->, but not have to use FK of E in D. And can't use Join table such as TABLE_D_E(d_id, e_id) used in the hibernate example.
If I dont need to navigate E->D, do I still need to define a <many-to-one> mapping in E.hbm.xml?

Looks like the FK E.d_id can only be mapped from within E.hbm.xml, and not inside D.hbm.xml?

I have been referring to example 7.2.2 at "http://www.hibernate.org/hib_docs/reference/en/html/assoc-unidirectional.html".

No, a <one-to-many> have to go in a <set> or other collection tag. For example:



By the way. Doesn't Hibernate complains when you have the <one-to-many> by itself. If I do like this I get the error message:

I have updated the example in my previous post to use "many-to-one", which is what is should have been, my apologies. Another way to put it is, in the example 7.2.2,
http://www.hibernate.org/hib_docs/refer ... ional.html
I want to change the schema from


to


But I want to be able to navigate from Person->Address. Similarly, D->E. What needs to change in the mapping(s) and the code? And do that using a foreign key, not join tables.

After the edit, I think nothing makes sense any more. Now you have what is essentially a <one-to-one> mapping between D and E.

What do you really want. Given a single D does it have one E or many E:s? And given a single E, does it have one D or many D:s?

After the edit, D to E is a one-to-one mapping. D is the parent, and it can have 0 or 1 instances of E. So I want to know what changes in the code for D, E, and the mappings D.hbm.xml and E.hbm.xml.