[kforge-dev] temporal object question

[John Bywater]

Hi Rufus,

Just on the topic of temporal properties and objects, I remembered you 
saying about SQLAlchemy et al words to the effect, "they all have 
version domain model." I can't seem to find where this is. Could you help?

Also, in my spare time I've taken my implementation of Fowler's patterns 
a little bit further. I thought you might be interested to know that I 
"figured out" that implementing temporal properties for one-many 
attributes seems to require maintaining a record of the history of the 
members of that collection, updated only when the property/object is 
"saved". Then you can just look up what the members of the association 
were, without somehow looking through the collected objects (which may 
or may not themselves be temporal...), and with decent performance. If 
you are implementing temporal one-many properties, you might need to do 
this to make it work well. It seemed wasteful at first, but now I don't 
think it is at all. Now I think it's just a different way of 
implementing one-many, that is where the one- has a list of references 
to collected objects, and those collected objects know nothing of being 
collected.

The other thing I decided for myself is that there seems to be a 
temporal domain of concern, which can be modelled. And I would think 
that a good implementation of such temporal aspects in a more concrete 
domain of concern would call upon (make use of) the temporal model (so 
these two concerns are separated).

So, unless a domain model needs to extend the temporal model, I don't 
think a normal domain model would itself derive objects from the 
temporal model, but instead would have attributes (inherited from and 
supported by the core) that are set according to the temporal aspects of 
the real domain.

Anyway, this is the temporal model I came up with in domain model:
http://project.knowledgeforge.net/domainmodel/trac/browser/trunk/src/dm/dom/temporal.py

It is directed by the registry, to reflect the registered model, pretty 
much from here:
http://project.knowledgeforge.net/domainmodel/trac/browser/trunk/src/dm/dom/registry.py#L64

And values are picked by the persistence model here:
http://project.knowledgeforge.net/domainmodel/trac/browser/trunk/src/dm/db.py#L771
http://project.knowledgeforge.net/domainmodel/trac/browser/trunk/src/dm/db.py#L867

It's fairly subtle, but pretty simple and very effective.

Anyway, I enjoyed working through those patterns. I think I understand 
them much better now. And I'm sure the code will enhance KForge, 
ScanBooker, Desire, Eternity, Provide, etc.

Best wishes,

John.


-- 
Appropriate Software Foundation
Registered in England and Wales
17 Chapel Street, Hyde Cheshire
Company number: 04977110
W: appropriatesoftware.net
T: 0870 720 2944
M: 0781 139 2292