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From "Pierre Lavignotte" <pierre.lavigno...@gmail.com>
Subject Re: Cayenne 101
Date Fri, 26 Dec 2008 19:19:23 GMT
Hi Joe,

Question 1 :

you can't call the object constructor. Use the DataContext method instead
because it will register the object too.

Question  2 :
It depends what you want !

DataContext dc = DataContext.

getThreadDataContext();
> E1.getE2List().remove(e2);
> dc.commitChanges();

This will remove the link between E1 and E2 but E2 will still exist.

DataContext dc = DataContext.

getThreadDataContext();
> dc.deleteObject(e2);
> dc.commitChangesToParent();
> dc.commitChanges();


This will delete E2... but I think it will still be present in
E1.getE2List() because the list can be in memory.
I know it's not logical but I'm quite sure I allready encontered this case.

I use Cayenne 2.0.4 so Cayenne 3.0 can differ in some points.

Pierre


Cordialement,
Pierre Lavignotte
Ingénieur Conception & Développement
http://pierre.lavignotte.googlepages.com


On Fri, Dec 26, 2008 at 6:32 PM, Joe Baldwin <jfbaldwin@earthlink.net>wrote:

> I have a few questions about best practices.   I have done some
> experimenting, have read what I could find, and have some questions about
> some elementary Cayenne usage concerning Add & Delete with a
> Parent-ChildList design.  (I have recently experienced some odd behavior
> that may be due to a fundamental misunderstanding of DataContext rules.)
>
> Environment: I am using 3.0 M4 with MySQL and Tomcat JSP
> DataContext:  I am getting the context using:
> DataContext.getThreadDataContext();
> Example Design:
>        E1 has a list of E2's
> that is:
>        E2 is a child of E1 and is a many-to-one relationship.
>        The E1 relationship is called "E2List" and the reverse relationship
> on E2 is called "E1".
>
> Questions:
> 1. When creating and associating E2 children is it more proper to do the
> following:
>
>        DataContext dc = DataContext.getThreadDataContext();
>        ** [get E1 via a  DataContext query]
>        E2 e2 = new E2();
>        e2.setE1(e1); // I **assume** this registers e2 with the Context &
> adds e2 to e1's list
>        dc.commitChanges();
>
> OR do you need to do as your example suggests:
>
>        DataContext dc = DataContext.getThreadDataContext();
>        E2 e2 = (E2) dc.newObject(E2.class);
>        e2.setE1(e1);
>        dc.commitChanges();
>
> OR is there a better way?
>
> 2. Removal of a Child
> Can you remove the child using:
>        DataContext dc = DataContext.getThreadDataContext();
>        dc.deleteObject(e2);
>        dc.commitChangesToParent();
>        dc.commitChanges();
>
> or must you do it via the parent:
>        DataContext dc = DataContext.getThreadDataContext();
>        E1.getE2List().remove(e2);
>        dc.commitChanges();
>
> (Note: the second method seems to work better.)
>
> Thanks,
> Joe
>
>

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