Another take on inheritance

[Andrus Adamchik <an...@objectstyle.org>]

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
  

Re: Another take on inheritance

[Andrus Adamchik <an...@objectstyle.org>]

On Sep 27, 2008, at 2:42 PM, Lachlan Deck wrote:

>>
>> 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.

This is a definition of OO inheritance in general - superclass  
"properties" are shared between all subclasses.

> 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?

True, that could be a possible strategy. Hard to say how it will  
perform in an "average case" (whatever that is), compared to the full  
resolving of objects via separate queries and subsequent merge. At the  
very minimum we'd need to optimize a very common special case of  
"fetch a list and iterate through all elements", to prevent massive  
faulting of individual objects. Some sort of smart batch faulting will  
be needed... Maybe we can use some of the methodologies developed for  
the paginated lists to optimize that. E.g. fetch all instances of a  
particular subclass in the list, once any object of that instance is  
requested from the list (and do that for both regular results and to- 
many relationship lists).

Andrus

Re: Another take on inheritance

[Lachlan Deck <la...@gmail.com>]

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