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From Aristedes Maniatis <...@ish.com.au>
Subject Re: "Can't get primary key from temporary id" :(
Date Wed, 22 Apr 2009 00:17:20 GMT

On 22/04/2009, at 2:58 AM, Robert Zeigler wrote:

> Hm.... have you considered using runtime relationships? (NOTE: I've  
> never played with this, myself, so take the next bit with a grain of  
> salt. :)
> In theory, it's entirely possible with cayenne to have a single  
> generic data object and then modify the mapping information at  
> runtime as necessary.
> So... I /think/ you could do something along the following lines:
>
> Register a global lifecycle listener that listens to whatever events  
> you need: in your case, I would say you probably need post-load and  
> pre-persist?
> And then you can add the relationship information that you need or  
> that cayenne needs?
> Then the info is there for cayenne to work with.  And then you can  
> use the generic writeProperty/writeRelationship methods to access  
> the information in those properties as needed.
> This might be a long shot...dunno... like I said... I haven't tried  
> it.  But if I wanted to have relationships without having  
> relationships, this is likely the approach I would start looking at.
>
> I'd be interested in hearing what others have to say... feasible  
> given the current cayenne stack?

Unless you have real database relations you aren't going to get any  
benefit of being able to use Cayenne to follow relations. At some  
point in Cayenne it will try to follow joins by using SQL JOINs.  
Unless Andrus has a trick for making this work, I suspect your  
approach is clever but ultimately will run into difficulties.

The problem is that the original poster is trying to use Cayenne like  
he uses pure SQL and he hasn't demonstrated how he would even achieve  
what he wants with pure SQL (hint: LAST_INSERT_ID() is the mysql  
specific way of getting the last id. His other approaches just don't  
work. But even LAST_INSERT_ID() would probably create race conditions  
with multiple concurrent users.)

There is however a solution to relationships or attributes which need  
to be added to 75% of the tables in the database: inheritance. In this  
case vertical inheritance is the appropriate choice. We are just now  
testing the excellent work Andrus has done in this area and we have  
all the basic parts of inheritance working in ROP.


Ari Maniatis


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