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Re: Object creation questions
On Tue, 04 Jul 2000, Charles Benett wrote:
> Falko, Lars, or someone,
>
> Can you explain about object creation with ozone in more detail? I'm
> spending ages going round in circles.
>
> Particular questions are:
> 1) Do all persistent classes need a constructor with no parameter and no
> body?
No. The compiler generates an default ctor if needed.
> 1b) If so, why doesn't the default constructor work?
see 1)
> 2) Do all persistent objects need to be created by calling the
> no-parameter, no-body constructor? or can you call another constructor?
There is a createObject method that calls other then the default ctors. But it
needs some more work because it needs the signature of the ctor as string.
Maybe someone can help and write some lines of code to generate the signature
out of the parameter objects.
> If you call another constructor does it have to call the ( ) {} one?
> 3) What is the difference between creating objects from outside the db
> server and creating them within the db server?
There is no difference.
...except that creating a database object from another database object is much
faster than from the client. And, if you are creating more than one objects at
the same time, doing this inside a database method encloses all the create
operations inside a transaction which is most likely wanted.
> 3b) I seem to be able to create db objects within other db objects with
> calls to new, rather than to .createObject() but their behaviour seems
> erratic (see below). Is this supposed to work or is it inadvisable?
Calling new does not create a database object. However, not all persistent
classes have to be database classes. A Car class may have a string member:
name. This name object is persistent and will be stored in the database but it
is not a database object because no other than the Car object can access it.
> 4) Am I right in thinking that if you create an object with
> database().createObject(myObject), the constructor of myObject can't
> create other objects - I get an "object is not yet associated to a
> database container" exception.
Yes. Therefore, although it doesn't look that nice, an initialization method is
always a good idea.
> 5) Can you explain about named objects, e.g. do names have to be unique
> within DB or only unique within a class of objects?
Names are unique within a database. Named objects are the entry points to your
data that allows you to access the stored data from a newly started client.
> Sorry for so many questions!
no problem.
> Thanks,
> Charles
>
> Here's an outline of what I want to do:
>
> Client: rootThingy = db.createObject(root_object_name);
> Client: rootThingy.addSomething_update(param);
>
> rootThingy.addSomething_update(param) {
> Something anObj = new Something(param);
> }
>
> where Something is an object I want to be persistent, with a constructor
> like:
>
> Something ( ) {
> myField = param;
> }
>
> Now, this seems to create the objects OK (from debug messages). Also, if
> I want to read myField later, I can navigate from root_object_name and
> get it OK.
> However, if I want to call a method of Something, it doesn't work.
>
> PS An example with more than two layers of db objects (ie more than
> garage and car), where objects at all layers are manipulated would be
> handy.
The DOM (XML) code is recursive. So you can have as many layers as you want.
But this doesn't change anything. Things are exactly the same between object of
level 4 and 5 and objects of level 1 and 2. In fact, there is no such thing
like the object level. Objects are database object or they are not. That's it.
Falko
--
______________________________________________________________________
Falko Braeutigam mailto:falko@softwarebuero.de
softwarebuero m&b (SMB) http://www.softwarebuero.de