[Newbies] Concrete classes... multiple users

Sun Oct 28 03:44:01 UTC 2007

Hi John,

Good luck with your project.  By the way if you haven't already heard about
it you might want to look at Glorp.  Glorp is an object to RDBMS mapping
system for Smalltalk (written in VW but ported to Squeak).  It is possible
that it is more complicated then you need but you may find that it is very
useful.  Reading the documentation may also give you some idea of the types
of issues that you may encounter in your system.  

It does sound like you are having fun.  There is nothing like solving real
problems for learning.

Happy coding!!

Ron

> -----Original Message-----
> From: John Almberg
> 
> Hi Ron,
> 
> In a nutshell, I have an application that needs to read Items from a
> database.
> 
> I recently discovered that in Smalltalk classes are real objects.
> This struck me as cool, so when I was deciding how to instantiate
> these Items, I thought, hey I'll use the class object as a factory
> (actually, I think it's more of an adapter pattern, but...)
> 
> So I taught the Item class object how to talk to the database,
> including username/password, and then I was able to do something like:
> 
> item = Item findById: 2
> 
> And items from the database were instantiated as Items, seemingly by
> magic.
> 
> This seemed so intuitive and simple, and just 'right'. It
> encapsulated and hid the whole legacy database messiness and adapted
> the database fields into the Item attributes I really needed.
> 
> Best of all, it could later be reprogrammed to talk to a slightly
> different database and adapt that database's fields into exactly the
> same Items, so the rest of the program didn't have to know it was
> talking to a different database.
> 
> The problem arose when I needed to talk to more than one database *at
> the same time*, since the Item class was programmed to talk to a
> specific database. The only way to talk to another database was to
> update the class variables before asking for the new Item. This was
> ugly and, worse, the Item users had to know that there were different
> databases to deal with.
> 
> Anyway, yesterday I realized I needed multiple objects to connect to
> multiple databases. I figured I had two choices:
>    - subclass the Item class
>    - refactor entirely and build something like an ItemAdapter class,
> who's instantiated objects would connect to the database, create Item
> objects, initialize them from the data records, and hand them to the
> Item consumers.
> 
> The second choice seemed the better, so that's what I did.
> 
> So, I think it was a case of falling in love with a cool feature, and
> then mis-applying it, just because it was dying to be used.
> 
> Hope this explains it... I'm afraid I don't always get the jargon
> right, but that's why I'm a newbie, I guess!
> 
> -- John
> 
> 
> 
> On Oct 26, 2007, at 10:48 PM, Ron Teitelbaum wrote:
> 
> > Hi John,
> >
> > I read your question, but had some trouble trying to understand
> > what you did
> > or why you are having trouble now that you have to connect to multiple
> > databases.
> >
> > You say that you have only one Item class.  That's good because
> > your domain
> > classes shouldn't be affected by your backend database.  You mention a
> > factory to decide on and build your adapter.  That's good too
> > because based
> > on some rule you would need to figure out what database to use.
> >
> > Then I get foggy, you say you have to change class variables.  Ok
> > so I'm
> > guessing that your factory sets the class variables which hold your
> > adaptors
> > that are used to pull items.
> >
> > I think your question is: how can I keep from having to change
> > database
> > adapter class variables?  If this is not your question please let
> > me know.
> >
> > The answer to that question is to modify your model to provide the
> > flexibility you need so that you can supply adaptors to your
> > application.
> > Don't use class variables, use some other object instead.  Classes
> > are cheap
> > if you need more functionality.
> >
> > There is some rule that you are modeling that tells you when you
> > need one
> > database versus another database.  You already have adapters that
> > handle the
> > different databases, so you only need to build a new object to hold
> > those
> > database sessions, built from your factory, and some session
> > manager that
> > knows when to attach and which adapter to use.
> >
> > So build a session manager, that holds multiple database sessions
> > (adapters), and code the logic for picking a session on the
> > manager.  When
> > global behavior stops being global move from class side (and class
> > variables) to a new class instance and model there instead.  Let me
> > know if
> > any of that wasn't clear, feel free to provide more specific
> > information
> > about what you are trying to do.
> >
> > Happy Coding!!
> >
> > Ron Teitelbaum
> > President / Principal Software Engineer
> > US Medical Record Specialists
> >
> > ________________________________________
> > From: John Almberg
> >
> > One of the cool things, I think, about Smalltalk is that Classes are
> > objects... i.e. concrete factories that can be modified at runtime to
> > produce objects in different ways, depending on the runtime
> > situation. I
> > just find this so much more intuitive, compared to how classes are
> > done in
> > other languages...
> >
> > But I think I may have used it in a naive way...
> >
> > For example, I needed an Adapter class that could produce standard
> > Item
> > objects by connecting to a variety of legacy database tables,
> > converting the
> > non-standard item records into standard Item objects. This way, the
> > consumer
> > of those Item objects doesn't care where the items come from.
> >
> > Originally, I thought I would only have to make these connections,
> > one at a
> > time. I.e., I'd program the Item class 'factory' with the legacy
> > database
> > hostname, username, password, and then just start asking for Items.
> > The Item
> > class would then go out to the database, fetch the proper row,
> > create an
> > Item object, initialize the instance variables, and return it to
> > the Item
> > object consumer.
> >
> > This is intuitive, cool, and it works.
> >
> > Then the requirements changed and I needed to connect to multiple
> > database
> > *at the same time*.
> >
> > But there is only one Item class object!
> >
> > This really muddles things up, because I basically have to update
> > the Item
> > class variables every time I need an Item object. No longer cool!
> >
> > Did I just misuse this feature? Should I have built two Adapter
> > Objects,
> > instead? Am I missing something obvious? Is a little knowledge a
> > dangerous
> > thing? :-)
> >
> > TIA... John
> >
> >
> > _______________________________________________
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> > Beginners at lists.squeakfoundation.org
> > http://lists.squeakfoundation.org/mailman/listinfo/beginners
> 
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