Re: Adding A New Component With A 1:1 Relationship

[Thomas Mahler]

Hi Michael,

Michael Duffy wrote:
I have a newbie question, if you don't mind.

Your advanced O/R tutorial shows a 1:1 mapping example
with Article and ProductGroup classes.  You've shown
the table schemas (in German, but I follow you) and
the repository_user.xml.  All very good, of course.  

The repository XML for Article has a productGroupId
that points to the ProductGroup key, and an instance
of ProductGroup that is supposed to be owned by the
Article.
Now if I want to ask the broker to store a brand new
Article, how do I do it?  Is the productGroupId
required to be set to an existing before I call
broker.store(newArticle)?  
Depends on the database. If there is a FK constraint then yes.
But as you can see from the table schema, there are no formal FK 
constraints. So, the productGroupId needs not to be filled.

If I want to have a new
ProductGroup for my Article, do I have to do this as a
five-step transaction?
1. Create a new ProductGroup instance
2. Store the new ProductGroup instance
3. Get the latest ProductGroup instances's primary key
4. Create a new Article, using the PG primary key
5. Store the new Article
No it's much easier, as OJB takes care of FK assigment automatically:
1. Create a new Article a
2. Create a new ProductGroup instance pg
3. call a.setProductGroup(pg)
4. store the new Article.
OJB manages the FK assignment for you. If you have set 
auto-update="true" on the productGroup reference-descriptor then OJB 
will also store the new PG on storing the new article!


I'm sure it's my lack of understanding, because this
feels very difficult.  I'm still climbing that OJB
learning curve.
I agree, there is a learning curve with OJB. O/R mapping is not trivial. 
But it is worth the effort. Once you mastered the learning curve you 
will ask yourself, how could I write database applications without an 
O/R tool?

cheers,
Thomas
Thank you for your work, help, and patience. 
Sincerely, MOD

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