Re: [Modeling-users] Re: Implementing inheritance through vertical mapping

[Federico H. <fh...@vi...>]

On Thu, 2003-09-25 at 10:37, Sebastien Bigaret wrote:
> I can't remember if it's been discussed here, however this is the way
> I think this could be done:
>  - models: add support for flattened attributes and
>    relationships. Flattened properties are exactly this: take the value
>    for a given field from another entity (hence potentially from another
>    table). Flattened properties will be declared with a simple path,
>    such as: toEmployee.toStore.name
>    (BTW, backward-compatibility should, and will, be ensured)

OK, so what we're talking about here is that the XML is a description of
the *classes*, and how they map to the database, not a description of
the database itself (i.e. "entities" declared in the XML are "class"
entities, not "table" entities). No problem with that, I'm just trying
to make sure we're talking about the same things. In this way of
handling things, the XML will never reflect inheritance, and all classes
that are part of an inheritance hierarchy will have to specify the
flattening of each attribute. This makes writing the XML by hand a major
pain in the neck, though it probably doesn't matter (at least to us,
we're planning on generating this XML from our own, richer XML dstabase
schema description).

>  - make the framework correctly handle those flattened properties. As
>    far as I can see, this would only impact the low-level bricks of the
>    framework (esp. SQL generation for fetching and saving).

I agree, this should be pretty straightforward, and could be implemented
without breaking anything... people who don't use it wouldn't even
notice it's there.=20

> Could you possibly elaborate on that? I never played with postgresql's
> inheritance support, and it's probably something that could be added in
> the User's Guide until vertical mapping is supported. If you find some
> time to illustrate the mechanisms, I'll be interested in learning how
> they behave wrt. the framework.

Well, I haven't done much work with it personally either, but one of my
pals here has. The story is that you can specify inheritance in a
PostgreSQL database scheme. When you tell PostgreSQL that table Mammal
inherits from Vertebrate, a "SELECT * FROM Mammal" will include all
fields defined in Vertebrate as well. The interesting part is that the
Vertebrate fields are actually stored in their own table... think of it
as vertical mapping implemented in the DBMS. So unique indentifiers are
unique across all Vertebrates, regardless of whether they are Mammals or
not.

I have not tried this yet, but it looks to me that this could be used to
put our application (which uses vertical mapping) in such a state that
it can be used with the current framework.

	Fede