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From: Lachlan Deck <lachlan.deck@gmail.com>
To: dev@cayenne.apache.org
In-Reply-To: <1839A675-E6E8-4194-8AF3-4281818C4CB4@objectstyle.org>
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Subject: Re: Another take on inheritance
Date: Sat, 27 Sep 2008 21:42:28 +1000
References: <1839A675-E6E8-4194-8AF3-4281818C4CB4@objectstyle.org>

On 26/09/2008, at 5:09 AM, Andrus Adamchik wrote:

> 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 :-)

This last one may not require separate queries for each subclass.  
i.e., by definition horizontal inheritance ensures that each sub- 
entity has the super's attributes/relations in common. So I'd think  
you ought to be able to do a unioned fetch of these things across the  
immediate child entities and lazy fault anything unique to the child  
entities thereafter... no?

with regards,
--

Lachlan Deck