Posted by Manuel Lemos on 09/20/05 17:17

Hello,

on 09/20/2005 04:59 AM Murray @ PlanetThoughtful said the following:
>>> Let's take for example a class called 'Customer' that (obviously)
>>> manipulates customers in the database. Here is a very basic Customer
>>> class. (Data validation and the like are left out for brevity.)
>> This is a basic object persistence problem.
>>
>>
>>> (Unless I've already got some major design flaws I think we should be
>>> good to go.)
>> >
>> > Where I get tripped up is when I realize I'll need to at some point
>> > get more than one customer at a time and thus I want to add a method
>> > called 'get_customers()'.
>>
>>
>> Yes, there is a problem. You are trying to retrieve objects into memory
>> before they exist. It makes more sense that you retrieve objects using a
>> factory class.
>>
>> That is the approach of Metastorage. You may want to take a looka at
>> Metastorage before you reinvent the wheel.
>
> Hi Manuel,
>
> I very much understand your desire to promote your various projects, but the
> original poster is asking a question that is basic to any programmer's
> development in object-oriented coding.

I denote a certain bias on your part against the fact that I suggest
projects that I have developed.

The matter is that I am suggesting projects that I use. The fact that I
developed them is a mere coincidence. I do not always suggest projects
that I developed as I am not such a wizard that I have the best
solutions for everything. However, if I use and recommend certain
solutions, mine or others is because I believe they are the best at
least for my purposes.

What the original poster wants to know is what is the best solution
retrieve a collection of objects. I recommended what I use and explained
why I think it is the best approach for this purpose. He appreciated the
reply and sent me a thank you message.

I do not understand why this could bother you or anybody else. If you
have a better solution, nothing stops you to make your recommendations.



> Once he understands how to solve class abstraction problems such as the one
> he is asking about, he will be better equipped to deal with a wider range of
> application development tasks.

Right, that is why I suggested something that I believe that solves his
probleme better and even justified.


> This is not to trivialize your Metastorage project (or, to be more accurate,
> I know nothing about it, so it's not my place to trivialize it or
> otherwise), but to point out that 'out-of-the-box' solutions to fundamental
> coding development problems probably ultimately makes for a poorer
> programmer. I could well be wrong, but it seems this is a case of "give a
> man a fish" as opposed to "teach a man to fish".

I think you should have learned about Metastorage first before
commenting. It is not really a out-of-the-box solution. It is a code
generator tool that employs well known design patterns to generate code
to customized to the developer needs.

It is not yet another template pasting tool. It uses the JE WIN
approach: Just Exactly What I Need. That works by letting the developer
tell the kind of functions he wants to use, rather than bundling a pile
of code that you may never user.

It takes in account the design of your persistent object classes to
generate optimized code for the task, rather than generic code for
unnecessary generic needs.

Basically it generates the code that you would generate if you were an
expert in object persistence methodology.

Given that, I am not even saying, "use this", but rather, take a look at
the kind of code that it generates as a concrete example of what I tried
to explain. The generated code even comes with comments so you do not
have to wonder too much.

Whether the original poster will use Metastorage or just pick some
ideas, that is another story.


> Also, and separate from above, I don't understand the relevance of your
> comment, "You are trying to retrieve objects into memory before they exist".
> Unless I'm horribly mistaken [1], the original poster has developed a class
> that abstracts a single customer, and is asking the list for suggestions in
> how to best approach a need to be able to abstract collections of customers.
> This is a normal application development issue, and for the life of me I
> can't grasp how your comment relates.

I tried to explain in the part of the message that you did not quote,
why using a factory class as entry point, which is my suggestion, it
makes more sense.

In case this was not very clear, who gives existence to the objects
should be a parent, not a brother like he was doing. He was using a
Customer class object to give birth to other unrelated Customer objects.

When using a factory class you can execute a query to retrieve Customer
objects and create them on demand. This means that you do not have to
create one Customer object first. The query may return data for 0 or
more Customer objects. A factory may create just exactly the number of
Customer objects as needed.

Furthermore, a factory class may solve the problem of avoiding to create
multiple copies of the same Customer object in memory. That can be done
by keeping a private array of all Customer objects retrieved in memory
so far.

While retrieving the Customer collection query results, the factory
class can check if a given customer object was already retrieve. If so
it may return a reference to previously retrieved object instead of
creating a new copy.

This is important because if you changed an Customer object in memory
and later query a collection of customer object, if a new object copy is
created it would be inconsistent with what you changed before. The
consequences for your application can be disastrous.

I am not sure if this explanation is clear now as I admit that these
matters are very abstract.


--

Regards,
Manuel Lemos

PHP Classes - Free ready to use OOP components written in PHP
http://www.phpclasses.org/

PHP Reviews - Reviews of PHP books and other products
http://www.phpclasses.org/reviews/

Metastorage - Data object relational mapping layer generator
http://www.meta-language.net/metastorage.html

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