=head1 NAME

ORM - Object relational mapper for Perl.

=head1 SYNOPSIS

Purpose of this document is to brief introduce usage of PerlORM library
on simple example. Example is 'Tasks Planner' (or 'Todo List') application.

=head1 LESSON 1: CREATING OBJECT MODEL

Let's start with simple object model, which will be improved
and modified as needed later.
Object classes of our example application is:

=over

=item 1. Task

Properties:

=over

=item * Title (title)

=item * Detailed description (desc)

=item * Task creation time (created)

=item * Task start time (start_date), can be undef

=item * Task end time (end_date), can be undef

=item * Task deadline (deadline), can be undef

=item * Responsible worker (worker)

=back

=item 2. Worker

Properties:

=over

=item * Worker name (name)

=back

=back

First step in creation of object model is to create so called
B<initial class>. Initial class is base class for all classes of our
object model.

File Todo/ORM.pm

  package Todo::ORM;

  use ORM::Db::DBI::MySQL;
  use base 'ORM';

  BEGIN
  {
      ORM->_init
      (
          prefer_lazy_load     => 0,
          emulate_foreign_keys => 1,
          default_cache_size   => 200,

          db => ORM::Db::DBI::MySQL->new
          (
              host        => 'localhost',
              database    => 'todo_list',
              user        => 'root',
              password    => '',
          ),
      );
  }

  1;

Now let's declare classes of our model.

PerlORM does not require to declare class properties in both
class declaration and database. Creation of database table 
for storing objects of the class is quite enough.
Fields in this table will correspond to object properties.

One or more database tables are assigned to each class
(more than one table is used in case of inheritance).
Each object of the class is represented by single row in table
or rows inner join in case of inheritance.

Initial declaration of classes looks very simple:

File Todo/Task.pm

   package Todo::Task;

   $VERSION=0.1;

   use ORM::Base 'Todo::ORM';

File Todo/Worker.pm

   package Todo::Worker;

   $VERSION=0.1;

   use ORM::Base 'Todo::ORM';

There is one question: how PerlORM detects what table to use for
certain class? If table name is not specified obviously then
ORM class calls method C<_guess_table_name> which is by default
uses regexp C<$class =~ s/::/_/g;> to detect table name from class
name. You can change this behaviour by overriding C<_guess_table_name>
method in your initial class. For example:

   sub _guess_table_name
   {
       my $my_class = shift;
       my $class = shift;
       my $table;

       $table = substr( $class, index( $class, '::' )+2 );
       $table =~ s/::/_/g;

       return $table;
   }

Now table for class C<Todo::Task> should be named C<Task> and not 
C<Todo_Task>.

It's time to create database tables.
(Name of database being used is specified in storage driver constructor.)

  CREATE DATABASE todo_list;

  DROP TABLE IF EXISTS `todo_list`.`_ORM_refs`;
  CREATE TABLE `_ORM_refs` (
      `class` varchar(45) NOT NULL default '',
      `prop`  varchar(45) NOT NULL default '',
      `ref_class` varchar(45) NOT NULL default '',
      PRIMARY KEY  (`class`,`prop`)
  ) TYPE=InnoDB;

  INSERT INTO '_ORM_refs' VALUES ( 'Todo::Task', 'worker', 'Todo::Worker' );

  DROP TABLE IF EXISTS `todo_list`.`Task`;
  CREATE TABLE `task` (
      `id` bigint(20) unsigned NOT NULL auto_increment,
      `title` varchar(255) NOT NULL default '',
      `desc` text NOT NULL,
      `created` date default NULL,
      `start_date` date default NULL,
      `deadline` date default NULL,
      `worker` bigint(20) unsigned default NULL,
      PRIMARY KEY  (`id`)
  ) TYPE=InnoDB;

  DROP TABLE IF EXISTS `todo_list`.`Worker`;
  CREATE TABLE `worker` (
      `id` bigint(20) unsigned NOT NULL auto_increment,
      `name` varchar(100) NOT NULL default '',
      PRIMARY KEY  (`id`)
  ) TYPE=InnoDB;

We just created 4 tables, first of them C<_ORM_refs> is special table.
ORM uses it to detect links between classes in our model and with
third party classes. In our model C<worker> property of class
C<Todo::Task> should be the reference to an object of class C<Todo::Worker>.
To tell it to ORM we should insert following row in C<_ORM_refs>:

  class       | prop      | ref_class
  ------------------------------------
  Todo::Task  | worker    | Todo::Worker

For frequently used classes there is another way to define relations
between objects, this way is to override C<ORM::_db_type_to_class>
method in our initial class. C<_db_type_to_class> method accepts
table field name and type as its arguments and returns class that
should be assigned to property.

The default behavior of C<_db_type_to_class> method defined in ORM class
is to assign classes L<ORM::Date> and L<ORM::Datetime> to properties
described by fields of type C<DATE> and C<DATETIME> respectively.

Every table that is used with ORM should have autoincremented field C<id>
which stored ID of objects of corresponding class.

=head1 LESSON 2: CREATING AND UPDATING OBJECTS

Creation of objects in ORM is performed by calling 'new' method.
For example let's create 'Worker' object:

  use Todo::Worker;

  $error  = ORM::Error->new;
  $worker = Todo::Worker->new
  (
      prop  => { name => 'E. Cartman' },
      error => $error,
  );

  print $error->text;

If 'new' operation will fail then C<$error> object will contain
information about occured errors.

Update of object is performed similarly:

  use Todo::Worker;

  $error  = ORM::Error->new;
  $worker = Todo::Worker->find_id( id=>1, error=>$error );
  $worker && $worker->update( prop=>{ name=> 'Eric Cartman' }, error=>$error );

  print $error->text;

Use of $error object is not necessary but strongly recomended.

Now to more easily manage objects of our model we will create two perl
scripts, one for create objects C<new.pl> and second for updating C<update.pl>

File new.pl

  #!/usr/bin/perl
  #
  # Use: perl new.pl <Class> <Prop1Name> <Prop1Value> <Prop2Name> <Prop2Value>...
  #
  # Class - Name of the class without 'Todo::' prefix.
  #

  use lib "lib";
  use lib "../ORM/lib";

  $nick  = shift;
  $class = "Todo::$nick";

  eval "require $class" or die $@;

  $error = ORM::Error->new;
  %prop  = @ARGV;
  $obj   = $class->new( prop=>\%prop, error=>$error );

  if( $obj )
  {
      print "New $nick was created with id:".$obj->id."\n" if( $obj );
      $obj->print;
  }

  print $error->text;

File update.pl

  #!/usr/bin/perl
  #
  # Use: perl update.pl <Class> <ObjectID> <Prop1Name> <Prop1Value> <Prop2Name> <Prop2Value>...
  #
  # Class - Name of the class without 'Todo::' prefix.
  #

  use lib "lib";
  use lib "../ORM/lib";

  $nick  = shift;
  $class = "Todo::$nick";

  eval "require $class" or die $@;

  $id    = shift;
  $error = ORM::Error->new;
  %prop  = @ARGV;
  $obj   = $class->find_id( id=>$id, error=>$error );

  if( $obj )
  {
      $obj->update( prop=>\%prop, error=>$error ) unless( $error->fatal );
      print "Updated $nick with id:".$obj->id."\n";
      $obj->print;
  }
  else
  {
      print STDERR "Object #$id of $class not found!\n";
  }

  print $error->text;

Both scripts use C<print> method we doesn't declare yet.
This method is aimed to print plain text information about specified
object. This method should be defined in initial class so every object
of our model can acces it.

  sub print
  {
      my $self  = shift;
      my $ident = shift||0;
      my @ref;

      # Do not dive deeper that third level of recursion
      # when printing information about related objects.

      return if( $ident > 3 );

      # Print information about specified object
      
      print ' 'x($ident*2),('-'x20),"\n";
      for my $prop ( (ref $self)->_all_props )
      {
          printf "%".(20+$ident*2)."s %s\n", "$prop:", $self->_property_id( $prop );
          if( (ref $self)->_prop_is_ref( $prop ) && $self->_property( $prop ) )
          {
              push @ref, $self->_property( $prop );
          }
      }
      print ' 'x($ident*2),('-'x20),"\n\n";

      # Print information about related objects

      for my $prop ( @ref )
      {
          print ' 'x(($ident+1)*2),"Related object '$prop':\n";
          $prop->print( $ident+1 );
      }
  }

Note the way properties were accessed.
For this purpose $obj->_property( $property_name ) were used in above code.
To access object properties with more grace there is AUTOLOAD method.
So you can simply call $obj->$property_name (or just $obj->deadline for example).
The C<ORM::_property> method is for cases when $property_name method 
should be redefined in child class for some reason.
Also you can use $obj->_property_id( $property_name ) to get raw database value
of the property. Its result is:

=over

=item * the same as of C<ORM::_property_id> for plain (non-object) properties

=item * database property value for non-ORM third party classes

=item * object's ID for ORM classes

=back

Now we can fill our model with some objects.

  # perl new.pl Worker name "Kenny McCormic"
  New Worker was created with id:2
  --------------------
                   id: 2
                class: Todo::Worker
                 name: Kenny McCormic
  --------------------

  # perl new.pl Task \
        title "Kill Kenny" \
        desc "Just kill Kenny!" \
        worker 1 \
        created "2005-12-18" \
        start_date "2006-01-01" \
        deadline "2006-01-02"

  New Task was created with id:1
  --------------------
                   id: 1
                class: Todo::Task
              created: 2005-12-18
                 desc: Just kill Kenny!
               worker: 1
             deadline: 2006-01-02
                title: Kill Kenny
           start_date: 2006-01-01
  --------------------

    Related object 'worker':
    --------------------
                     id: 1
                  class: Todo::Worker
                   name: Eric Cartman
    --------------------

  # perl new.pl Task \
        title "Eat Chocolate pie" \
        desc "Ask your mummy." \
        worker 1 \
        created "2005-12-18" \
        start_date "2006-01-01" \
        deadline "2006-01-02"

  New Task was created with id:2
  --------------------
                   id: 2
                class: Todo::Task
              created: 2005-12-18
                 desc: Ask your mummy.
               worker: 1
             deadline: 2006-01-02
                title: Eat Chocolate pie
           start_date: 2006-01-01
  --------------------

    Related object 'worker':
    --------------------
                     id: 1
                  class: Todo::Worker
                   name: Eric Cartman
    --------------------

For more comfort let's modify C<Todo::Task> class so
it can assign current time to C<created> property when
explicit value is not specified:

  sub _validate_prop
  {
      my $self = shift;
      my %arg  = @_;

      if( ! $self->id && ! $self->created )
      {
          $self->_fix_prop
          (
              prop  => { created=>ORM::Date->current },
              error => $arg{error},
          );
      }

      $self->SUPER::_validate_prop( %arg );
  }

=over

=item * Method C<_validate_prop> is implicitly called when new object
is being created (C<new> method) and when object is being updated
(C<update> method).

=item * Condition ( !$self->id ) means than object is not yet stored in
database table and therefore doesn't have ID assigned to it.
In another words this means than condition will be true only in C<new> method.

=item * Method '_fix_prop' is intended to use only within C<_validate_prop>.

=item * Do not forget to call C<SUPER::_validate_prop>.

=back

Let's add one more task:

  # perl new.pl Task \
        title "Keep alive" \
        desc "Just keep alive!" \
        worker 2 \
        start_date "2005-12-31" \
        deadline "2006-01-02"

  New Task was created with id:3
  --------------------
                   id: 3
                class: Todo::Task
              created: 2005-12-18
                 desc: Just keep alive!
               worker: 2
             deadline: 2006-01-02
                title: Keep alive
           start_date: 2005-12-31
  --------------------

    Related object 'worker':
    --------------------
                     id: 2
                  class: Todo::Worker
                   name: Kenny McCormic
    --------------------

As you can see C<created> property is implicitly initialized
with default value of current time.
(It seems like Kenny will die anyway after deadline.)

=head1 LESSON 3: SELECTING AND FILTERING

Now when we have some tasks planned for workers
it's time to make some reports about tasks state.
Interesting reports are:

=over

=item * Tasks planned to be done by specific worker

=item * Tasks that should be done due specified date

=back

Tasks for first report can be selected as follows:

  ORM::DbLog->write_to_stderr( 1 );
  @tasks = Todo::Task->find
  (
      filter => ( Todo::Task->M->worker == $worker ),
      error  => $error,
  );

Todo::Task->M->worker - is so named B<Meta-property>, Meta-property
is object of class C<ORM::Metaprop> or its descendants.
In resulting SQL-query Meta-properties are replaced with
names of corresponding table fields.
Special meta-property Todo::Task->M means object of class <Todo::Task> itself.
Below you will see that meta-properties is very powerful facility
and is also easy to use.

Variable $worker should contain C<Todo::Worker> object
or just its integer ID.

Variable $error of type C<ORM::Error> will contain description of
error if any occured during query.
C<error> parameter is not required, if it is omitted then
error is silently ignored. In future version this behavious can 
be changed.

Call C<ORM::DbLog>->C<write_to_stderr( 1 )> enables trace of so called
B<SQL-log> to STDERR this is useful tool for debugging you code.
In described case (assuming $worker=1) SQL-log trace will look as follows:

  --------------------------
  [Mon Dec 26 00:14:27 2005]: ORM::find: Success
  SELECT
    DISTINCT `Task`.*
  FROM
    `Task`
  WHERE
    (`worker` = '1')

If we need to select tasks by worker name, then method call
will look like this:

  @tasks = Todo::Task->find
  (
      filter => ( Todo::Task->M->worker->name eq $worker_name ),
      order  => ORM::Order->new( [ Todo::Task->M->created, 'DESC' ] ),
      error  => $error,
  );

Draw attention on using of operators C<==> and C<eq>. Databases usually
have no sence to this operator because in most cases they will be
translater to SQL C<=> operator which is used for string and
numeric comparisons. Nevertheless for best readability it is reasonable to
use this operators as in native Perl.

Parameter 'order' specifies that found tasks should be sorted by C<created>
time in descendant order.

Let's try little more complicated query when we need to find tasks
assigned to workers containing some string in their names:

  @tasks = Todo::Task->find
  (
      filter => ( Todo::Task->M->worker->name->_like( '%Cartman%' ) ),
      order  => ORM::Order->new( [ Todo::Task->M->created, 'DESC' ] ),
      error  => $error,
  );

Resulting SQL-query for the call:

  SELECT
    DISTINCT `_T1_Task`.*
  FROM
    `Task` AS `_T1_Task`
      LEFT JOIN `Worker` AS `_T2_Worker` ON( `_T1_Task`.`worker`=`_T2_Worker`.`id` )
  WHERE
    (`_T2_Worker`.`name` LIKE '%Cartman%')
  ORDER BY `_T1_Task`.`created` DESC

Call for second report looks much similar:

  $M     = Todo::Task->M;
  @tasks = Todo::Task->find( filter => ( $M->deadline < '2006-01-30' ) );

Variable $M is for brevity, such trick is useful when constructing complex
meta-expressions.

There is another interesting report about number of tasks assigned
to each worker, for this report we will use C<stat> method,
This method is useful when you need info about related objects:

  $M   = Todo::Worker->M;
  $res = Todo::Worker->stat
  (
      data =>
      {
          worker => $M,
          tasks  => $M->_rev( 'Todo::Task' => 'worker' )->_count,
      },
      group_by => [ $M ],
      preload  => { worker=>1 },
  );

Opposite to C<find> method which returns array of objects C<stat>
method returns array of hashes with requested data.

Parameter C<data> is hash reference that defines what kind of data
should be retrieved from database. Resulting hash will contain
records with exactly the same keys as in C<data> parameter and
with values retrieved from database as specified by values of C<data>.

In out case $res will contain hashes with two keys C<worker> - 
C<Todo::Worker> object and C<tasks> - number of assigned tasks.

Parameter C<group_by> similar to SQL C<GROUP BY> statement.
In resulting SQL-query C<group_by> will be replaced with C<GROUP BY>.
It is used to define how to apply grouping method C<_count>.

Parameter C<preload> defines objects that should be loaded by resulting 
query and not later by separate query.

Meta-property $M->_rev( 'Todo::Task' => 'worker' ) so called 
B<reversive meta-property>. It is used to access objects that refer
to selected objects by one of its property. In our case objects of
class C<Todo::Task> referring to objects of class C<Todo::Worker>
by property C<worker>, therefore we can reversively access tasks
assigned to a worker.

SQL-query for the call:

  --------------------------
  [Mon Dec 26 00:49:34 2005]: ORM::stat: Success
  SELECT
    'Todo::Worker' AS `_worker class`,
    COUNT( `_T2_Task`.`id` ) AS `tasks`,
    `_T1_Worker`.`id` AS `worker`,
    `_T1_Worker`.`name` AS `_worker name`
  FROM
    `Worker` AS `_T1_Worker`
      LEFT JOIN `Task` AS `_T2_Task` ON( `_T1_Worker`.`id`=`_T2_Task`.`worker` )
  GROUP BY `_T1_Worker`.`id`

=head1 LESSON 4: DELETING OBJECTS

Method 'delete' is used for deletion of objects from database.

  $worker->delete( error=>$error );

If 'emulate_foreign_keys' option to C<ORM::_init> method is
set to true then before deletion ORM will check if there are
another objects that refer to object being deleted.
If so $error object will contain corresponding error message.

After object $worker has been deleted from database it is reblessed
to be object of class C<ORM::Broken>. Call to any method of
this object will croak with error message. This is to be sure
that object is not being used after it has been deleted.

=head1 LESSON 5: OBJECT CHANGES HISTORY

To enable built-in feature to trace object changes
simply use C<history_class> argument to C<ORM::_init>
method as shown below:

  ORM->_init
  (
      history_class => 'Todo::History',
      %another_args,
  );

Next step is to declare C<Todo::History> class. This class
will behave as any other regular ORM-class of our model.
This means that any change of any object will be introduced
by one or more 'Todo::History' object.
History class declaration is quite simple:

File Todo/History.pm

  package Todo::History;

  $VERSION=0.1;

  use ORM::Base 'Todo::ORM', i_am_history=>1;
  
  1;

As we have redefined C<ORM::_guest_table_name> method,
the table for C<Todo::History> class will have assumed
name 'History'. Structure of the table should be as
follows:

  DROP TABLE IF EXISTS `todo_list`.`History`;
  CREATE TABLE `History` (

      `id`        bigint(20)   NOT NULL auto_increment,

      `obj_class` varchar(100) NOT NULL default '',
      `obj_id`    int(11)      NOT NULL default '0',

      `prop_name` varchar(100) NOT NULL default '',
      `old_value` varchar(255)          default '',
      `new_value` varchar(255)          default '',

      `date`      datetime     NOT NULL,
      `editor`    varchar(255) NOT NULL default '',
      `slaved_by` bigint(20) unsigned   default NULL,

      PRIMARY KEY  (`id`)

  ) TYPE=InnoDB;

From this moment every 'new' (SQL INSERT), 'update' (SQL UPDATE)
and 'delete' (SQL DELETE) actions will be logged in 'History' table.

Each action will be stored in at least one history object.
'new' action create exactly one C<Todo::History> object.
'update' and 'delete' actions will create more than one C<Todo::History> objects.
'update' action will create number of additional history objects
equal to number of properties affected by operation.
'delete' action will create number of additional history objects
equal to number of properties of deleted object.

All history object created for 'delete' or 'update' actions execpt first
object will have property 'slaved_by' set to id of first history object,
this object is called 'master'.

Main use of History class is to store all changes made to objects
of our model. But there is one more interesting feature. You can
undo changes with 'Todo::History::rollback' method.
For example you can restore deleted object by its ID like this:

  $hist = Todo::History->find
  (
      filter => ORM::Expr->_and
      (
          # rollback operation should be called on master history object
          Todo::History->M->master, 
          Todo::History->M->obj_class eq 'Todo::Worker',
          Todo::History->M->obj_id == 1,
      ),
  );
  $hist && $hist->rollback;

This code will do the following:

=over

=item 1. Find 'slaved' history objects

=item 2. Create 'Todo::Worker' object with properties from 'slaved' history objects

=item 3. Delete 'slaved' and 'master' history objects

=back

Also it is possible to restore object not by ID but by arbitrary property
like this:

  $hist = Todo::History->find
  (
      filter => ORM::Expr->_and
      (
          Todo::History->M->obj_class eq 'Todo::Worker',
          Todo::History->M->prop_name == 'name',
          Todo::History->M->old_value->_like( '%Cartman%' ),
          Todo::History->M->delete_slave,
      ),
  );
  $hist && $hist->slaved_by->rollback;

It is possible to rollback 'new' and 'update' actions by similar fashion.
You can find more detailed description of this feature in L<ORM::History>.

=head1 LESSON 6: ERROR HANDLING

Error handling is done by passing 'error' argument to almost
every ORM method. 'error' argument should contain object of class
C<ORM::Error>. Consider the following code:

  use Todo::Task;

  $task  = Todo::Task->find; # first found task
  $error = ORM::Error->new;
  $task->update
  (
      prop  => { worker=>-1 },
      error => $error,
  );
  print STDERR "Failed to update\n" if( $error->fatal );
  print STDERR $error->text;

Output for this code:

  Failed to update
  fatal: ORM->_normalize_prop_to_db_value(): Property 'worker' of type 'Todo::Worker' with id='-1' was not found  

Classes of our object model also can produce their own errors and send them to the caller.

For example we should not permit to set Todo::Task->deadline property to be less than current date.
To implement this functionality we will modify C<Todo::Task> class by adding new condition to
C<_validate_prop> method:

  if( $self->deadline < ORM::Datetime->current )
  {
      $arg{error}->add_fatal( "Deadline is in past!" );
  }

As far as C<_validate_prop> method is called only implicitly
from 'new' and 'delete' methods, you can be sure that $arg{error}
contains correct C<ORM::Error> object. That's why there is noreason
to test it before use.

In cases when you not sure that user has passed $error object to your method
you can use following scheme:

  sub my_metod
  {
      my $self  = shift;
      my %arg   = @_;
      my $error = ORM::Error->new;

      $error->add_fatal( "test 1" );
      $error->add_warn( "test 2" );

      $error->upto( $arg{error} );
      return undef;
  }

Call to 'upto' method sends error back to user if $arg{error}
contains valid error-object.

=head1 LESSON 7: CACHING

=head2 ORM-object caching

Object's in-memory cache implemented on per-primary-class basis.
B<Primary class> - is class that is a direct descendant of B<initial class>.

The idea of such cache strategy is to have several caches,
by one for every primary class. Objects of non-primary
class will use cache of its primary class. With that in
mind its time to say that C<default_cache_size> argument
to C<ORM::_init> specifies cache size for one
primary class and not total cache size.

In our object model this means that there is maximum
of 200 cached objects for C<Todo::Worker> class
and 200 cached objects for C<Todo::Task> class.

You can change cache size of individual primary class
this way:

  package Todo::Task;
  
  $VERSION=0.1;

  use ORM::Base 'Todo::ORM', cache_size=>1000;

You can get current efficiency of in-memory cache
by calling ORM::Cache->total_efficiency and 
reset the cache counters by calling ORM::Cache->clear_stat

When C<default_cache_size> is set to zero, it is still
possible that object will be loaded from cache.
This is because of B<instance caching> (see below).

Cache is organized in the fashion when you
do not need to purge it manually.

=head2 Perl-object caching

This mechanism is mostly common for ORMs and persistence tools in Perl.
This means that after

  $task1 = Todo::Task->find_id( id=>1 );
  $task2 = Todo::Task->find_id( id=>1 );

$task1 and $task2 will contain the same blessed reference.
And even in the following case:

  $task1 = Todo::Task->find_id( id=>1 );
  $task2 = Todo::Task->find( order=>[ Todo::Task->M->id ] );
  $task3 = Todo::Task->find( filter=>( Todo::Task->M->desc->_like( %kill Kenny% ) ) );

All three variables will contain the same object.

Perl-object caching will work even when cache size is set to 0.

=head2 Properties caching

When you first access some object-property of ORM-object, then
ORM tries to create expected object for you (using find_id for 
ORM-object properties and __ORM_new_db_value for non-ORM-properties)
and then store it in internal object's hash.

When you access the same object-property later it will not be
created again but fetched from hash.

When you need to refresh object's contents simply call

  $task->refresh;

=head1 LESSON 8: TRANSACTIONS

The usual way transactions should be used is to
set up a correspondence between single method and
single transaction. If it is not the case then probably
there is some conceptional error in object model.

  package Todo::Worker;

  # Delegate all worker's tasks to new worker
  sub delegate_tasks
  {
      my $self  = shift;
      my %arg   = @_;
      my $error = ORM::Error->new;
      my $ta    = Todo::ORM->new_transaction( error=>$error );

      for my $task ( $self->_rev_prop( 'Todo::Task'=>'worker', error=>$error ) )
      {
          $task->worker( new_value=>$arg{worker}, error=>$error );
      }

      $error->upto( $arg{error} );
  }

As you can see the way transactions is used in ORM is different
than commonly used in other packages. In ORM you do not need to 'finish'
transaction explicitly. This helps you to avoid situations when you forget
to call method to finish transaction. $ta contains the object assigned to
a newly started transactions. Transaction is finished when $ta object
is destroyed. In our case this happens when method is finished.

If $error contain one or more fatal errors when
$ta is being destroyed, then transaction will be rolled back.
Also transaction rollback is performed for active transaction
when die function called or runtime error occured.

Note use of '_rev_prop' method in foreach loop:

  $self->_rev_prop( 'Todo::Task'=>'worker', error=>$error );

is the same as:

  Todo::Task->find( filter=>( Todo::Task->M->worker==$self ), error=>$error );

Transactions in ORM can be nested:

  package Todo::Worker;
  
  # Delegate all old worker's tasks to new worker
  # and delete old worker
  sub discharge
  {
      my $self  = shift;
      my %arg   = @_;
      my $error = ORM::Error->new;
      my $ta    = Todo::ORM->new_transaction( error=>$error );

      $self->delegate_tasks( worker=>$arg{worker}, error=>$error );
      $self->delete( error=>$error );

      $error->upto( $arg{error} );
  }

As you can see transactional method 'delegate_tasks'
is called from another transactional method 'discharge'.
Nested transactional object simply will not issue
"BEGIN TRANSACTION" to the SQL-server.
In case when nested transaction is failed then outer
transaction is failed too and therefore is rolled back.

=head1 LESSON 9: LAZY LOAD (To be documented)

=head1 LESSON 10: INHERITANCE (To be documented)


=head1 LESSON 11: SOME REFACTORING (To be documented)


=head1 SEE ALSO

http://www.sourceforge.net/projects/perlorm

=head1 AUTHOR

Alexey V. Akimov

=head1 COPYRIGHT AND LICENSE

Copyright (C) 2005 by Alexey V. Akimov

This library is free software; you can redistribute it and/or modify
it under the terms of LGPL licence.

=cut

