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From Lachlan Deck <lachlan.d...@gmail.com>
Subject Re: Vertical inheritance
Date Thu, 03 Jun 2010 02:43:24 GMT
On 01/06/2010, at 11:38 PM, Andrus Adamchik wrote:

> On May 31, 2010, at 9:11 AM, Andrus Adamchik wrote:
> 
>> BTW, semantically "vertical inheritance with discriminator" is essentially single-table
inheritance with flattened attributes in subclasses. Which Cayenne supports already, but without
any special optimizations for wide|deep hierarchies.
> 
> Pounding on this idea some more ... Since we can't get away from using entity qualifier
(discriminator) at least in some cases for performance reasons, and I hate to add multiple
strategies, maybe we do make the qualifier required and treat vertical as a special case of
single table with subclasses mapped to the same root table, and having flattened attributes
mapped to subclass-specific table. The benefits of that are:
> 
> * No implicit inheritance relationship from super to sub table. It is explicitly mapped
inside flattened attributes.
> * More intuitive mapping, easier to visualize attributes, as all attributes are rooted
in the same base table.
> * Can potentially handle more than one joined table per subclass, or the same join table
for multiple subclasses, or a mix of single table mapping with joined table mapping. I.e.
in the spirit of Cayenne, we'd allow users to follow a generic DB semantics in their mapping
instead of forcing an arbitrary ORM concepts on a (legacy) DB schema.
> * No new concepts for the backend or Modeler to deal with.

+1

> Now we still need to do some work with this design:
> 
> * Make sure SELECT/INSERT/DELETE/UPDATE work correctly with flattened attributes over
1..1 relationships, and especially when inheritance is involved.
> * Add convenience Modeler methods to flatten all attributes at once for a given relationship
to simplify subclass mapping.
> * Add performance optimizations per Mike's idea, limiting the number of joins done in
a single query.
> 
> Mike, do you see any holes in this design?

Looks good.

with regards,
--

Lachlan Deck


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