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From Andrus Adamchik <and...@objectstyle.org>
Subject Another take on inheritance
Date Thu, 25 Sep 2008 19:09:37 GMT
No guarantee that it won't end up being just another mailing list  
discussion with no code committed, but giving it another shot. Myself  
and Dzmitry Rusak (he hasn't participated on this list, but hopefully  
he will over time; he is the guy who developed DataMap generator  
templates that create named query methods) have been looking at the  
CAY-795 Jira and Marcin's patch to implement horizontal inheritance,  
with the goal to advance this task further.

We discussed ORM inheritance in general, possible approaches to the  
mapping, and just touched a bit on the runtime. For me it was helpful  
to realize that there is no spoon :-) For all the talk about single- 
table, vertical and horizontal inheritance, Cayenne runtime will end  
up dealing with a hybrid case of any possible combination of the above  
in each given hierarchy. So the terminology will be different when we  
get to nuts and bolts of the mapping: ObjEntity to DbEntity mapping;  
ObjEntity qualifier; ObjAttribute overrides in subclasses. These are  
the three pieces that allow inheritance to happen in the mapping (all  
3 types of inheritance).

Let me come back to this in a moment... Now a note on implementation.  
I'd like to follow an approach that proved itself with other features:

(1) implement new features needed in the mapping layer (including save/ 
load)
(2) implement Modeler support for that (although that can be postponed  
till after #3 as to avoid early exposing of the incomplete features to  
the end users)
(3) implement runtime support for select / select relationship /  
insert / update / delete, with unit tests
(4) do #3 for ROP

So essentially this is going layer by layer instead of cutting across,  
and when we get to runtime parts, we already have a mapping framework  
to create the test cases. The plan is to bypass Marcin's patch (that  
has code across the layers) and start with (1) and (2). For #1, some  
preliminary work has been done by Ari and Marcin (CAY-794), so now we  
need to relax some mapping restrictions (coming from assuming that all  
inheritance == single table inheritance) and introduce a new concept  
of ObjAttribute overrides. It exists in JPA (for the same purpose of  
mapping inheritance), and so far has been absent from Cayenne (except  
for the embeddables that are still somewhat hidden from the public  
view by the absent Modeler support).

Once we have that, we can move to the runtime pieces... That's where  
the real fun starts... Consider that to fetch superclass in case of  
horizontal inheritance, we'd need to do separate queries on each  
subclass on the tree, and then combine the result in an in-memory  
"union" and then apply in-memory sort ordering on top of that :-)

Andrus
  

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