NAME

Class::PObject - Framework for programming persistent objects

SYNOPSIS

After loading the Class::PObject with use, we can declare a pobject like so:

pobject Person => {
    columns     => ['id', 'name', 'email'],
    datasource  => './data'
};

We can also declare the pobject in its own .pm file:

package Person;
use Class::PObject;
pobject {
    columns     => ['id', 'name', 'email''
    datasource  => './data'
};

We can now create an instance of above Person, and fill it in with data, and store it into disk:

$person = new Person();
$person->name('Sherzod');
$person->email('sherzodr@cpan.org');
$new_id = $person->save()

We can access the saved Person later, make necessary changes and save back:

$person = Person->load($new_id);
$person->name('Sherzod Ruzmetov (The Geek)');
$person->save()

We can load multiple objects as well:

@people = Person->load();
for ( $i = 0; $i < @people; $i++ ) {
    $person = $people[$i];
    printf("[%02d] %s <%s>\n", $person->id, $person->name, $person->email)
}

or we can load all the objects based on some criteria and sort the list by column name in descending order, and limit the results to only the first 3 objects:

@people = Person->load(
                {name => "Sherzod"},
                {sort => "name", direction => "desc", limit=>3});

We can also seek into a specific point of the result set:

@people = Person->load(undef, {offset=>10, limit=>10});

WARNING

This is 'alpha' release. I mainly want to hear some ideas, criticism and suggestions from people. Look at TODO section for more details.

DESCRIPTION

Class::PObject is a class framework for programming persistent objects. Such objects can store themselves into disk, and recreate themselves from the disk.

If it is easier for you, just think of a persistent object as a single record of a relational database:

+-----+----------------+--------+----------+
| id  | title          | artist | album_id |
+-----+----------------+--------+----------+
| 217 | Yagonam O'zing | Sevara |        1 |
+-----+----------------+--------+----------+

The above record of a song can be represented as a persistent object. Using Class::PObject, you can define a class to represent this object like so:

pobject Song => {
    columns => ['id', 'title', 'artist', 'album_id']
};

Now you can create an instance of a Song with the following syntax:

$song = new Song(title=>"Yagonam O'zing", artist=>"Sevara", album_id=>1);

All the disk access is performed through its drivers, thus allowing your objects truly transparent database access. Currently supported drivers are mysql, file and csv. More drivers can be added, and I believe will be.

PROGRAMMING STYLE

The style of Class::PObject is very similar to that of Class::Struct. Instead of exporting 'struct()', however, Class::PObject exports 'pobject()' function. Another visual difference is the way you declare the class. In Class::PObject, each property of the class is represented as a column.

DEFINING OBJECTS

Object can be created in several ways. You can create the object in its own .pm file with the following syntax:

package Article;
use Class::PObject;
pobject {
    columns => ['id', 'title', 'date', 'author', 'source', 'text']
};

Or you can also create an in-line object - from within your programs with more explicit declaration:

pobject Article => {
    columns => ['id', 'title', 'date', 'author', 'source', 'text']
};

Effect of the above two examples is identical - a class representing an Article. By default, Class::PObject will fall back to file driver if you do not specify any drivers. So the above Article object could also be redefined more explicitly like:

pobject Article => {
    columns => \@columns,
    driver => 'file'
};

The above examples are creating temporary objects. These are the ones stored in your system's temporary location. If you want more permanent objects, you should also declare its datasource:

pobject Article => {
    columns => \@columns,
    datasource => './data'
};

Now, the above article object will store its objects into data/article/ folder. Since data storage is so dependant on the drivers, you should consult respective driver manuals for the details of data storage-related topics.

Class declarations are tightly dependant to the type of driver being used, so we'll leave the rest of the declaration to specific drivers. In this document, we'll concentrate more on the user interface of the Class::PObject - something not dependant on the driver.

CREATING NEW OBJECTS

After you define a class using pobject(), as shown above, now you can create instances of those objects. Objects are created with new() - constructor method. To create an instance of the above Article object, we do:

$article = new Article()

The above syntax will create an empty Article object. We can now fill columns of this object one by one:

$article->title("Persistent Objects with Class::PObject");
$article->date("Sunday, June 08, 2003"),
$article->author("Sherzod B. Ruzmetov");
$article->source("lost+found (http://author.handalak.com)");
$article->text("CONTENTS OF THE ARTICLE GOES HERE");

Another way of filling in objects, is by passing column values to the constructor - new():

$article = new Article(title  =>  "Persistent Objects with Class::PObject",
                       date   =>  "Sunday, June 08, 2003",
                       author =>  "Sherzod Ruzmetov",
                       source =>  "lost+found (http://author.handalak.com" );

$article->text("CONTENTS OF THE ARTICLE GO HERE");

Notice, above example is initializing all the properties of the object except for text in the constructor, and initializing text separately. You can use any combination to fill in your objects.

STORING OBJECTS

Usually, when you create the objects and fill them with data, they are in-memory data structures, and not attached to disk. This means as soon as your program terminates, or your object instance exits its scope the data will be lost. It's when you call save() method on the object when they are stored in disk. To store the above Article, we could just say:

$article->save();

save() method returns newly created object id on success, undef on failure. So you may want to check its return value to see if it succeeded:

$new_id = $article->save() or die $article->errstr;

Note: we'll talk more about handling exceptions in later sections.

LOADING OBJECTS

No point of storing stuff if you can't retrieve them when you need them. PObjects support load() method which allows you to re-initialize your objects from the disk. You can retrieve objects in many ways. The easiest, and the most efficient way of loading an object from the disk is by its id:

$article = Article->load(1251);

the above code is retrieving an article with id 1251. You can now either display the article on your web page:

printf("<h1>%s</h1>",  $article->title);
printf("<div>By %s</div>", $article->author);
printf("<div>Posted on %s</div>", $article->date);
printf("<p>%s</p>", $article->text);

or you can make some changes, say, change its title and save it back:

$article->title("Persistent Objects in Perl made easy with Class::PObject");
$article->save();

Other ways of loading objects can be by passing column values, in which case the object will retrieve all the objects from the database matching your search criteria:

@articles = Article->load({author=>"Sherzod Ruzmetov"});

The above code will retrieve all the articles from the database written by "Sherzod Ruzmetov". You can specify more criteria to narrow your search down:

@articles = Article->load({author=>"Sherzod Ruzmetov", source=>"lost+found"});

The above will retrieve all the articles written by "Sherzod Ruzmetov" and with source "lost+found". We can of course, pass no arguments to load(), in which case all the objects of the same type will be returned.

Elements of returned @array are instances of Article objects. We can generate the list of all the articles with the following syntax:

@articles = Article->load();
for my $article ( @articles ) {
    printf("[%02d] - %s - %s - %s\n",
            $article->id, $article->title, $article->author, $article->date)
}

load() also supports second set of arguments used to do post-result filtering. Using these sets you can sort the results by any column, retrieve first n number of results, or do incremental retrievals. For example, to retrieve first 10 articles with the highest rating (assuming our Article object supports rating column):

@favorites = Article->load(undef, {sort=>'rating', direction=>'desc', limit=>10});

The above code is applying descending ordering on rating column, and limiting the search for first 10 objects. We could also do incremental retrievals. This method is best suited for web applications, where you can present "previous/next" navigation links and limit each listing to some n objects:

@articles = Article->load(undef, {offset=>10, limit=>10});

Above code retrieves records 10 through 20. The result set is not required to have a promising order. If you need a certain order, you have to specify sort argument with the name of the column you want to sort by.

@articles = Article->load(undef, {sort=>'title', offset=>10, limit=>10});

By default sort applies an ascending sort. You can override this behavior by defining direction attribute:

@articles = Article->load(undef, {sort=>'title', direction=>'desc'});

You can of course define both terms and arguments to load():

@articles = Article->load({source=>'lost+found'}, {offset=>10, limit=>10, sort=>'title'});

If you load() objects in array context as we've been doing above. In this case it returns array of objects regardless of the number of objects retrieved.

If you call load() in scalar context, regardless of the number of matching objects in the disk, you will always retrieve the first object in the data set. For added efficiency, Class::PObject will add limit=>1 argument even if it's missing.

COUNTING OBJECTS

Counting objects is very frequent task in many programs. You want to be able to display how many Articles are in a web site, or how many of those articles have 5 out of 5 rating.

You can of course do it with a syntax similar to:

@all_articles = Article->load();
$count = scalar( @all_articles );

But some database drivers may provide a more optimized way of retrieving this information using its meta-data. That's where count() method comes in:

$count = Article->count();

count() also can accept \%terms, just like above load() does as the first argument. Using \%terms you can define conditional way of counting objects:

$favorites_count = Article->count({rating=>'5'});

The above will retrieve a count of all the Articles with rating of '5'.

REMOVING OBJECTS

PObjects support remove() and remove_all() methods. remove() is an object method. It is used only to remove one object at a time. remove_all() is a class method, which removes all the objects of the same type, thus a little more scarier.

To remove an article with id 1201, we first need to create the object of that article by loading it:

# we first need to load the article:
my $article = Article->load(1201);
$article->remove();

remove() will return any true value indicating success, undef on failure.

$article->remove() or die $article->errstr;

remove_all() is invoked like so:

Article->remove_all();

Notice, it's a static class method.

remove_all() can also be used for removing objects selectively without having to load them first. To do this, you can pass \%terms as the first argument to remove_all(). These \%terms are the same as the ones we used for load():

Article->remove_all({rating=>1});

DEFINING METHODS OTHER THAN ACCESSORS

In some cases you want to be able to extend the class with custom methods.

For example, assume you have a User object, which needs to be authenticated before they can access certain parts of the web site. It may be a good idea to add "authenticate()" method into your User class, which either returns the User object if he/she is logged in properly, or returns undef, meaning the user isn't logged in yet.

To do this we can simply define additional method, authenticate(). Consider the following example:

package User;

pobject {
    columns     => ['id', 'login', 'psswd', 'email'],
    datasource  => 'data/users'
};

sub authenticate {
    my $class = shift;
    my ($cgi, $session) = @_;

    # if the user is already logged in, return the object:
    if ( $session->param('_logged_in') ) {
        return $class->load({id=>$session->param('_logged_in')})
    }

    # if we come this far, we'll try to initialize the object with CGI parameters:
    my $login     = $cgi->param('login')    or return 0;
    my $password  = $cgi->param('password') or return 0;

    # if we come this far, both 'login' and 'password' fields were submitted in the form:
    my $user = $class->load({login=>$login, psswd=>$password});

    # if the user could be loaded, we set the session parameter to his/her id
    if ( defined $user ) {
        $session->param('_logged_in', $user->id)
    }
    return $user
}

Now, we can check if the user is logged into our web site with the following code:

use User;
my $user = User->authenticate($cgi, $session);
unless ( defined $user ) {
    die "You need to login to the web site before you can access this page!"
}

printf "<h2>Hello %s</h2>", $user->login;

Notice, we're passing CGI and CGI::Session objects to authenticate(). You can do it differently depending on the tools you're using.

ERROR HANDLING

PObjects try never to die(), and lets the programer to decide what to do on failure, (unless of course, you insult it with wrong syntax).

Methods that may fail are the ones to do with disk access, namely, save(), load(), remove() and remove_all(). So it's advised you check these methods' return values before you assume any success. If an error occurs, the above methods return undef. More verbose error message will be accessible through errstr() method. In addition, save() method should always return the object id on success:

my $new_id = $article->save();
unless ( defined $new_id ) {
    die "couldn't save the article: " . $article->errstr
}

Article->remove_all() or die "couldn't remove objects:" . Article->errstr;

MISCELLANEOUS METHODS

In addition to the above described methods, PObjects also support the following few useful ones:

columns() - returns hash-reference to all the columns of the object. Keys of the hash hold column names, and their values hold respective column values:
```
my $columns = $article->columns();
while ( my ($k, $v) = each %$columns ) {
    printf "%s => %s\n", $k, $v
}
```
dump() - dumps the object as a chunk of visually formatted data structure using standard Data::Dumper. This method is mainly useful for debugging.
errstr() - class method. Returns the error message from last I/O operations, if any. This error message is also available through $CLASS::errstr global variable:
```
$article->save() or die $article->errstr;
# or
$article->save() or  die $Article::errstr;
```
__props() - returns class properties. Class properties are usually whatever was passed to pobject() as a hashref. This information is usually useful for driver authors only.
__driver() - returns either already available driver object, or creates a new object and returns it. Although not recommended, you can use this driver object to access driver's low-level functionality, as long as you know what you are doing. For available driver methods consult with specific driver manual, or contact the vendor.

TODO

Following are the lists of features and/or fixes that need to be applied before considering the library ready for production environment. The list is not exhaustive. Feel free to add your suggestions.

MORE FLEXIBLE LOAD()

load() will not be all we need until it supports at least simple joins. Something similar to the following may do:

@articles = Article->load(join => ['ObjectName', \%terms, \%args]);

I believe it's something to be supported by object drivers, that's where it can be performed more efficiently.

GLOBAL DESCTRUCTOR

PObjects try to cache the driver object for more extended periods than pobject's scope permits them to. So a global desctuctor should be applied to prevent unfavorable behaviors, especially under persistent environments, such as mod_perl or GUI.

Global variables that may need to be cleaned up are:

$Class::PObject::Driver::$drivername::__O: Where $drivername is the name of the driver used. If more than one driver is used in your project, more of these variables may exist. This variable holds particular driver object.
$PObjectName::props: Holds the properties for this particular PObject named $PObjectName. For example, if you created a pobject called Article, then it's properties are stored in global variable $Aritlce::props.

For example, if our objects were using just a mysql driver, in our main application we could've done something like:

END {
    $Class::PObject::Driver::mysql::__O = undef;
}

DRIVER SPECIFICATIONS

Class::PObject::Driver, Class::PObject::Driver::DBI

AUTHOR

Sherzod B. Ruzmetov, <sherzod@cpan.org>, http://author.handalak.com/

COPYRIGHT AND LICENSE

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

To install Class::PObject, copy and paste the appropriate command in to your terminal.

cpanm

cpanm Class::PObject

CPAN shell

perl -MCPAN -e shell
install Class::PObject

For more information on module installation, please visit the detailed CPAN module installation guide.

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