Hello Georg,
This may not be appropriate for your entire problem, but for the
example you've given, I think I would just model it to take
examination versions into account. Something like:
Exam -->> ExamVersion -->> Question
Having:
ExamVersion -->> StudentExam -->> Answer
and:
Answer -> Question
Then you could build in some mechanism for duplicating an exam to
create a starting point for a new version. So each exam a student
takes is tied to a version, which itself won't be modified, just new
versions added as necessary.
Hope this helps!
Regards,
Mark
On Jun 19, 2006, at 10:02 AM, Georg von Bülow wrote:
Hi,
I am facing a general design problem in EOF, maybe someone has
already
thought about it and could give me some hints.
I´m trying to implement a way of keeping track of "historic" data
in the
database with WebObjects.
That is, each time some data are changed, I would like to keep a
copy of
the old data with a status "historic" in the database, and insert
the new
(altered) data with status "active" in the database, instead of
overwriting and saving.
To illustrate the idea let me give a simple example.
I have an application with "examination results" of students.
These examinations were obtained through a questionnaire, which
consists
of many questions. If later I want to modify a question (or delete
one or
add a new one) for a future examination, I would like to archive
the old
questionnaire with its questions. Like this it is possible to
reproduce an
examination with its "historic" questionnaire and questions.
So I want to make a historic backup when:
one question is changed, one question is added, one question is
removed, a
questionnaire is changed etc...
I tried to implement this desired feature in EOF in a general
way. My
dream would be to have some kind of *framework* which I could
always add
(with a minimum of necessary adaptations) to already existing
applications, introducing the possibility of archiving data.
Here is the approach I have taken so far:
Entities who should be archived will obtain a field "archivestatus"
which
could be "active" or "historic".
I use my own extended EOEditingContext. To pick up only active data
when a
fetch is made, I overwrote objectsWithFetchSpecification() adding a
qualifier with "status = active", in case the entity can be
archived.
Then, when I want to save changes of a "question", I interfere in
saveChanges() of my EditingContext.
I have to make a copy of the altered question and its relating
objects. (I
use the copy()-method implemented in "Practical WebObjects", which
always
copies the whole "cycle" of related objects). The new objects are
set to
"active" and the old ones "historic".
Now if I remove or add one question it becomes messier. I would
have to
interfere in removeObjectFromBothSidesOfRelationshipWithKey(),
not to mention the complications when you add some objects and
change
others at the same time and then call saveChanges()...
Anyway, I feel that it´s getting much more complex than I would have
liked. I have some doubts if I´m going in the right direction at
all, or
is it just a too dangerous way to interfere like this in EOF!?
Maybe there is a clever pattern using EOObserverCenter?
Hope I have made clear enough my problem and how I tried to
approach it.
Any advice and comments are appreciated!
Greetings,
Georg
--------------------
Georg von Bülow
email@hidden
Niterói - Rio de Janeiro - Brasil