Re: Persistence

While relational models focus on flexibility in how they represent
information, object models don't benefit from this flexibility. There
is no single object model that can express enough semantic precision
for all applications.

Indeed.

Object models aren't supposed to capture all the
relationships among the information components, only those that are
relevant to the business context of a particular application.

I don't know if "supposed" is the correct word. I think "isn't able to"
is more correct. If an object model has several disadvantages compared
to a relational schema, why use it at all? How do we know an object
model is flexibale enough for the particular business context?

Of
course, several alternative object models can be assembled from
components in the same relational model.

And several alternative relational schemas may be mapped to the same
object model.

So think of the relational
database as a store of information components, as an engine for
querying those components, for maintaining integrity constraints across
those components, for supporting transactional updates of those
components, and think of an object model as one of a number of possible
assemblies of those components.

So, you use a relational database for other things but persistence?

There are likely a few cases where it makes sense to persist objects
directly, but I think those are relatively rare.

If you don't need anything but persistence, persisting objects directly
would make sense. But mostly you really need the other features
provided by a RDBMS.

Fredrik Bertilsson
http://frebe.php0h.com

.

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