eZ Components - PersistentObject
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. contents:: Table of Contents

Introduction
============
The PersistentObject component provides object persistence for PHP 5 using a
database. PersistentObject uses the Database component to provide database
abstraction. It does not rely on code generation and does not force a specific
inheritance structure to work.

PersistentObject is built to be fast and flexible, allowing you to build
persistent classes in the same way as any other class in your application. It
also utilizes the query abstraction layer of the Database component to make it
easy to build queries. Please refer to the API documentation of ezcQuerySelect
for detailed examples on how to use the query abstraction layer.

The PersistentObjectDatabaseSchemaTiein component allows you to automatically
generate the definition files needed by PersistentObject from a database schema
or the structure of your database. For more information, please refer to
the API documentation on ezcPersistentObjectSchemaGenerator.

Class overview
==============

ezcPersistentSession is the main API for interacting with the persistent
objects. Loading, saving and deleting persistent objects is done through
this class.

Basic usage
===========

This chapter describes the typical usage of PersistentObject with a
single persistent class, using MySQL as the persistence storage.

The persistent class
--------------------

We want to make a simple class, representing a person, persistent using
a persistent object. This is a simple class with only a few members: ::

    <?php
    class Person
    {
        private $id = null;
        public $name = null;
        public $age = null;

        public function getState()
        {
            $result = array();
            $result['id'] = $this->id;
            $result['name'] = $this->name;
            $result['age'] = $this->age;
            return $result;
        }

        public function setState( array $properties )
        {
            foreach( $properties as $key => $value )
            {
                $this->$key = $value;
            }
        }
    }
    ?>

The id member will map to the required persistent identifier. It has to default
to null. This is not required for any of the other mapped members. The id field
is a required unique identifier for this persistent object. It is generated by
the identifier generator and usually maps to an auto increment column in the
database.

For simplicity, we have made the name and age members of the Person class
public. However, this is not required and in a real application you can use 
any access method you like. You can even make the data completely private.

All persistent objects must implement the getState() and setState()
methods. They are used to retrieve the state of the object when saving it and
to set it when loading it. The getState() method should always return the
complete state of the object while the setState() method should set
only one member at the time.


The persistence mapping
-----------------------

We are going to map the Person class onto the following SQL table: ::

  CREATE TABLE persons
  (
    id integer unsigned not null auto_increment,
    full_name varchar(255),
    age integer,
    PRIMARY KEY (id)
  ) TYPE=InnoDB;

The fields map one to one to the members of the Person class. Using the
InnoDB type is not required. We strongly recommend it however, since it
supports transactions. The id column is of the type auto_increment. This is
required for the id generator that we will use. Other id generators may have
other requirements to work as expected.

In order for PersistentObject to be able to store objects of the Person class
into the persons table, we need to tell it how the columns are mapped to class 
members. We will use ezcPersistentCodeManager to fetch the definitions when
required.

ezcPersistentCodeManager requires us to define the mapping using the
ezcPersistentObjectDefinition, ezcPersistentObjectIdProperty and
ezcPersistentObjectProperty classes: ::

    <?php
    $def = new ezcPersistentObjectDefinition();
    $def->table = "persons";
    $def->class = "Person";

    $def->idProperty = new ezcPersistentObjectIdProperty;
    $def->idProperty->columnName = 'id';
    $def->idProperty->propertyName = 'id';
    $def->idProperty->generator = new ezcPersistentGeneratorDefinition( 'ezcPersistentSequenceGenerator' );

    $def->properties['name'] = new ezcPersistentObjectProperty;
    $def->properties['name']->columnName = 'full_name';
    $def->properties['name']->propertyName = 'name';
    $def->properties['name']->propertyType = ezcPersistentObjectProperty::PHP_TYPE_STRING;

    $def->properties['age'] = new ezcPersistentObjectProperty;
    $def->properties['age']->columnName = 'age';
    $def->properties['age']->propertyName = 'age';
    $def->properties['age']->propertyType = ezcPersistentObjectProperty::PHP_TYPE_INT;

    return $def;
    ?>

The first block of code creates the definition object and sets the database
table and the name of the class to map. The second block defines the mapping of
the identifier member and the algorithm that should be used to create
identifiers for new objects. We will use ezcPersistentSequenceGenerator,
which simply retrieves the new id generated by auto_increment.

The next two code blocks define the mapping between the database columns and
the class members. It is possible to use the same name in the class and the
database for a field.

The members must be inserted into the properties member, which is an
associative array, using the name of the member as the key name.
If you look at the API for ezcPersistentObjectDefinition,
it also has a property named "columns" that is the same array as the
"properties", except that it is mapped to the column names instead of the property
names. This reverse mapping is set up by ezcPersistentCodeManager.

Finally, we return the complete definition. Your definition will not work unless
you return it to the manager.

To make the definition work with the ezcPersistentCodeManager, it must be put in
a separate PHP file and given the name of the class in lowercase letters. In
our example, the filename should be person.php.

The session object
------------------

The session object is in charge of the actual loading and saving of persistent
objects. A session can be created by simply instantiating it: ::

    <?php
    $session = new ezcPersistentSession(
        ezcDbInstance::get(),
        new ezcPersistentCacheManager( new ezcPersistentCodeManager( "path/to/definitions" ) )
    );
    ?>

The session takes two arguments: a pointer to the database instance to use and
the manager from which to retrieve persistent object definitions. We are
using ezcPersistentCodeManager to load the definitions directly from
file. You should point it to the location where you saved the person.php
file. If you have several directories containing definitions, you can use
the ezcPersistentMultiManager class to add as many as you like. In addition to
the code manager we use a cache manager. The cache manager makes sure the
definition is loaded from disk only once.

While it is possible to create a new session each time you want to manipulate a
persistent object, you will probably want to use the same session each
time. This functionality can be achieved by using the
ezcPersistentSessionInstance class: ::

   <?php
   $session = new ezcPersistentSession( ezcDbInstance::get(),
                                        new ezcPersistentCacheManager( new ezcPersistentCodeManager( ... ) ) );
   ezcPersistentSessionInstance::set( $session ); // set default session

   // retrieve the session
   $session = ezcPersistentSessionInstance::get();
   ?>

Creating and updating an object
-------------------------------

Creating a new Person object and making it persistent is straightforward: ::

    <?php
    $object = new Person();
    $object->name = "Guybrush Threepwood";
    $object->age = 31;

    $session->save( $object );
    ?>

This code saves our newly created object to the database and generates an id
for it. The id is set to the id property of the object. Since Guybrush is our
first person, he is assigned the id of 1.

Of course, the age of Guybrush Threepwood is the source of much debate, and he
is probably younger than 31. To change his age, simply edit the object and
tell the session to update it. ::

    <?php
    $object->age = 25;
    $session->update( $object );
    ?>

Note that we used update() to store the object this time. This is because we
want to trigger an UPDATE query instead of an INSERT query.

Finding objects
----------------

There are several ways to retrieve persistent objects from the database. The
simplest is to fetch one object by its identifier. ::

    <?php
    $object = $session->load( 'Person', 1 );
    ?>

This code retrieves the Guybrush object created above.

If you have stored a lot of persistent objects to the database and want to retrieve a
list, you can use the find() method. The find() method requires a query parameter
that can first be retrieved from the session. ::

    <?php
    $q = $session->createFindQuery( 'Person' );
    $q->where( $q->expr->gt( 'age', $q->bindValue( 15 ) ) )
       ->orderBy( 'full_name' )
       ->limit( 10 );
    $objects = $session->find( $q, 'Person' );
    ?>

This code will fetch a maximum of 10 Person objects where the age is
higher than 15, sorted by name.

This is achieved by manipulating the query object returned by
ezcPersistentSession->createFindQuery(). To learn more about query abstraction
and how to use it, please refer to the `specific subsection of the Database
components tutorial`_.

.. _`specific subsection of the Database components tutorial`: introduction_Database.html#query-abstraction-usage

The find() method will fetch the complete result set and instantiate it for
you. This is not desirable if you are fetching large numbers of objects and you
want it to be fast and efficient. For this you can use the findIterator()
method: ::

    <?php
    $q = $session->createFindQuery( 'Person' );
    $q->where( $q->expr->gt( 'age', $q->bindValue( 15 ) ) )
      ->orderBy( 'name' )
      ->limit( 10 );
    $objects = $session->findIterator( $q, 'Person' );

    foreach( $objects as $object )
    {
        // ...
    }
    ?>

This code will produce the same result as the first find() example. However,
only one object will be instantiated and the data will be transferred from the
database only when it is needed.

The final example uses a find query with a logical and to find objects: ::

    <?php
    $q = $session->createFindQuery( 'Person' );
    $q->where( 
        $q->expr->lAnd( 
           $q->expr->eq( 'name', $q->bindValue( 'Guybrush Threepwood' ) ), 
           $q->expr->eq( 'age', $q->bindValue( 25 ) ) 
        )
    );
    $objects = $session->findIterator( $q, 'Person' );
    foreach( $objects as $object )
    {
        // ...
    }
    ?>

Deleting objects
-----------------

The easiest way to delete persistent objects is to use the delete() method on
the session: ::

    <?php
    $object = $session->load( 'Person', 1 );
    $session->delete( $object );
    ?>

Of course, you can only delete instantiated objects this way. If you want to
delete an object or a whole series of objects that are not instantiated, you can
use the deleteFromQuery() method: ::

    <?php
    $q = $session->createDeleteQuery( 'Person' );
    $q->where( $q->expr->gt( 'age', $q->bindValue( 15 ) ) );
    $session->deleteFromQuery( $q );
    ?>

The above code will remove all persons from the database who are more than 15
years old.


Identifier generation
=====================

All persistent objects must have an identifier field. The identifier
generation algorithm defines how the system will generate ids for new
objects. This chapter describes the available generators.

ezcPersistentSequenceGenerator
------------------------------

The sequence generator relies on the PDO::lastInsertId() method to retrieve the
ids for newly created persistent objects.

For databases supporting auto_increment (like MySQL and SQLite), use
ezcPersistentNativeGenerator. Other databases must use a sequence. For example,
the PostgreSQL person table definition should be as follows: ::

  CREATE TABLE persons
  (
    id integer unsigned not null,
    full_name varchar(255),
    age integer,
    PRIMARY KEY (id)
  );
  CREATE SEQUENCE person_seq START 1;

If your database requires you to use a sequence, this parameter should be
provided to ezcPersistentSequenceGenerator in the mapping definition. ::

    <?php
    $def->idProperty->generator = new ezcPersistentGeneratorDefinition(
        'ezcPersistentSequenceGenerator',
        array( 'sequence' => 'person_sequence' )
    );
    ?>

ezcPersistentNativeGenerator
----------------------------

The native generator relies on auto_increment, which is supported by, among
others, MySQL and SQLite. An example table definition looks like this: ::

    CREATE TABLE persons
    (
        id integer unsigned not null auto_increment,
        full_name varchar(255),
        age integer,
        PRIMARY KEY (id)
    );

The corresponding generator definition is below: ::

    <?php
    $def->idProperty->generator = new ezcPersistentGeneratorDefinition(
        'ezcPersistentNativeGenerator'
    );
    ?>

ezcPersistentManualGenerator
----------------------------

If you do not rely on a database mechanism to generate values for a primary
key column, you have to use the ezcPersistentManualGenerator class. You can
then set the value of the id property by hand and save the object afterwards.

For example: ::
    
    CREATE TABLE persons
    (
        login varchar(25),
        full_name varchar(255),
        age integer,
        PRIMARY KEY (login)
    );

In this table, the string value is used as the primary key. Therefore, we have
to generate id values manually. Use the following definition: ::

    <?php
    $def->idProperty->generator = new ezcPersistentGeneratorDefinition(
        'ezcPersistentManualGenerator'
    );
    ?>

For saving a new instance, use the following code: ::
    
    <?php
    $object = new Person();

    // Manually set the id
    $object->login = "guybrush";

    $object->name = "Guybrush Threepwood";
    $object->age = 31;

    $session->save( $object );
    ?>

Definition loaders
===================

The session object needs to be able to fetch the persistent object
definitions in order to function properly. The task of fetching the definitions
is performed by a definition loader, which is provided to the session when it is
instantiated. 

ezcPersistentCodeManager
-------------------------

This is currently the only manager available. It simply reads the definition
from a file with the same name as the class from the specified directory. It does
not perform any error checking on the definition and simply assumes that it is
correct.


Extending the definition loader
-------------------------------

It is very easy to create your own definition loader. Simply extend the
ezcPersistentDefinitionManager abstract class and implement the
fetchDefinition() method: ::

    <?php
    class ezcPersistentCodeManager extends ezcPersistentDefinitionManager
    {
        public function fetchDefinition( $class )
        {
        }
    }
    ?>

The fetchDefinition() method should create the definition structure for the
requested class or throw an exception.

Relations
=========

Relations are defined within the persistence mapping.

Relation class overview
-----------------------

The following definition classes are available to realize object relations:

ezcPersistentOneToManyRelation
  This class is used to define 1:n relations. For example, one person might be
  related to multiple addresses, but one address might only be related to
  one person.

ezcPersistentManyToManyRelation
  Using this class, you can define n:m relations. For example, a person can be
  related to multiple addresses, while an address can be related to multiple
  persons.

ezcPersistentOneToOneRelation
  With this class you can define 1:1 relations, which might be useful for
  slight de-normalization. For example, you can split your user data
  from the user credentials.

ezcPersistentManyToOneRelation
  This relation (n:1) does not make sense on its own, but as the reverse
  connection for a 1:n relation.

All of these classes extend the abstract class ezcPersistentRelation.

Relations basics
----------------

For the examples in this section, we will reuse the Person class, defined in
`The persistent class`_. In addition, we will use an Address class, which looks
as follows::

    <?php
    class Address
    {
        public $id = null;
        public $street = null;
        public $zip = null;
        public $city = null;

        public function setState( array $state )
        {
            foreach ( $state as $key => $value )
            {
                $this->$key = $value;
            }
        }

        public function getState()
        {
            return array(
                "id" => $this->id,
                "street" => $this->street,
                "zip" => $this->zip,
                "city" => $this->city,
            );
        }
    }
    ?>

The Address class will be extended later on to include relations. The
following basic persistence mapping is used and extended for each example::

    <?php
    $def = new ezcPersistentObjectDefinition();
    $def->table = "addresses";
    $def->class = "Address";

    $def->idProperty                = new ezcPersistentObjectIdProperty;
    $def->idProperty->columnName    = 'id';
    $def->idProperty->propertyName  = 'id';
    $def->idProperty->generator     = new ezcPersistentGeneratorDefinition( 'ezcPersistentSequenceGenerator' );

    $def->properties['street']                 = new ezcPersistentObjectProperty;
    $def->properties['street']->columnName     = 'street';
    $def->properties['street']->propertyName   = 'street';
    $def->properties['street']->propertyType   = ezcPersistentObjectProperty::PHP_TYPE_STRING;

    $def->properties['zip']                 = new ezcPersistentObjectProperty;
    $def->properties['zip']->columnName     = 'zip';
    $def->properties['zip']->propertyName   = 'zip';
    $def->properties['zip']->propertyType   = ezcPersistentObjectProperty::PHP_TYPE_STRING;

    $def->properties['city']                 = new ezcPersistentObjectProperty;
    $def->properties['city']->columnName     = 'city';
    $def->properties['city']->propertyName   = 'city';
    $def->properties['city']->propertyType   = ezcPersistentObjectProperty::PHP_TYPE_STRING;

    return $def;
    ?>

Defining a simple relation
--------------------------

The following extensions are necessary for the given class and persistence
mapping, to realize a simple 1:n relation. Each person will be able to have
multiple addresses, but one address may only refer to one person.

The Address class needs to be enhanced as follows, to store the id of the
Person it is related to. ::

    <?php
    class Address
    {
        // ...

        private $person;

        // ...

        public function getState()
        {
            return array(
                // ...
                "person"    => $this->person,
            );
        }
    }
    ?>

Additionally, we need to define the new property $person in the persistence
mapping of the Address class: ::

    <?php
    // ...

    $def->properties['person'] = new ezcPersistentObjectProperty;
    $def->properties['person']->columnName   = 'person_id';
    $def->properties['person']->propertyName = 'person';
    $def->properties['person']->propertyType = ezcPersistentObjectProperty::PHP_TYPE_INT;

    ?>

The relation definition takes place in the persistence mapping of the Person
class in `The persistent class`_. It needs to be extended as follows::

    <?php
    // ...

    $def->relations["Address"] = new ezcPersistentOneToManyRelation(
        "persons",
        "addresses"
    );
    $def->relations["Address"]->columnMap = array(
        new ezcPersistentSingleTableMap(
            "id",
            "person_id"
        )
    );

    // ..
    ?>

A relation to another persistent object is defined in the property
ezcPersistentObjectDefinition $relations, which is an array. Each relation must
have the name of the persistent object class it refers to as the key in this
array. An instance of one of the classes shown in `Class overview`_ must be the
value. In this case, it is ezcPersistentOneToManyRelation. The parameter to its
constructor are the names of the tables that the relation refers to. The first
table is the table of the current object, and the second one refers to the related
object.

To define which properties are used to realize the relation mapping, the
property ezcPersistentOneToManyRelation->columnMap is used. It contains an
array of (in this case) ezcPersistentSingleTableMap, which maps one column of
each of the tables to one column of another. In the above case, the
database column "id" from the table "persons" would be mapped to the column
"person_id" in the table "addresses". In general, this means, that "id" is the
primary key from the "persons" table and "person_id" is the foreign key in the
"addresses" table, that refers to the "persons" table.

If you want to map using several columns, you can add more
ezcPersistentSingleTableMap instances to the columnMap array. For example, if
you are using a person's first and last name as the primary key for the
"persons" table, you could define the relation like this: ::

    <?php
    // ...

    $def->relations["Address"] = new ezcPersistentOneToManyRelation(
        "persons",
        "addresses"
    );
    $def->relations["Address"]->columnMap = array(
        new ezcPersistentSingleTableMap(
            "firstname",
            "person_firstname"
        ),
        new ezcPersistentSingleTableMap(
            "lastname",
            "person_lastname"
        )
    );

    return $def;
    ?>

Using a relation
----------------

To use the previously defined 1:n relation, ezcPersistentSession offers several
new methods:

ezcPersistentSession->getRelatedObject()
  This method can be used to retrieve a single related object to a given source
  object. If no related object can be found, it will throw an exception.

ezcPersistentSession->getRelatedObjects()
  In contrast to ezcPersistentSession->getRelatedObject(), this method always
  returns an array of all related objects. It will not throw an exception if no
  related object can be found, but will instead return an empty array.

ezcPersistentSession->addRelatedObject()
  Using this method, you can build a relation between two persistent objects. It
  will set the defined properties on the objects, but does not store them to
  the database automatically. (Exceptions are
  ezcPersistentManyToManyRelation objects. Further details are below.)

ezcPersistentSession->removeRelatedObject()
  As the counterpart to ezcPersistentSession->addRelatedObject(), this method
  is used to remove the relation between two objects. It will not store the given
  objects for you, but only remove the necessary properties. (Again, exceptions
  are ezcPersistentManyToManyRelation objects. Further details are
  below.)

Using these methods, we can now retrieve all addresses that are related to one
person::

    <?php
    $person = $session->load( "Person", 1 );
    $addresses = $session->getRelatedObjects( $person, "Address" );
    ?>

The variable $addresses will then contain an array of all Address objects found
for the Person object with an id of 1. To relate these addresses to another
Person object, we can do the following::


    <?php
    $personOld = $session->load( "Person", 1 );
    $personNew = $session->load( "Person", 23 );
    $addresses = $session->getRelatedObjects( $personOld, "Address" );

    foreach ( $addresses as $address )
    {
        $session->removeRelatedObject( $personOld, $address );
        $session->addRelatedObject( $personNew, $address );
        $session->update( $address );
    }
    ?>

Defining n:m relations
----------------------

The eczPersistentManyToManyRelation class works slightly different than
the other ezcPersistentRelation classes. For this kind of relation, you need an
extra table in your database to store the relation records. The next example
shows the definition of an ezcPersistentManyToManyRelation relation, based on
the Person and Address example classes. Each person can have several
addresses and each address can be used by several persons. You need to use the 
original example classes for this, thus we do not need to extend them here. 
The definition of the relational mapping for the Person class must be extended 
as follows::

    <?php
    // ...

    $def->relations["Address"] = new ezcPersistentManyToManyRelation(
        "persons",
        "addresses",
        "persons_addresses"
    );
    $def->relations["Address"]->columnMap = array(
        new ezcPersistentDoubleTableMap( "id", "person_id", "address_id", "id" )
    );

    return $def;
    ?>
    
In contrast to all other implementations of ezcPersistentRelation, the
ezcPersistentManyToManyRelation constructor expects three table names:

- the name of the current objects table
- the name of the related objects table
- the table name to store the relation records

A similar exception applies to columnMap of this relation definition. It
consists of ezcPersistentDoubleTableMap instances, which carry four column names
each. The first column is the column to choose from the source table (usually
its primary key). The second column defines the column in your relation table
that maps to the first column. In our example, the column "id" from the
"persons" table maps to the column "person_id" from the relation table
"persons_addresses". The same applies to the third and fourth columns. The third
column defines the column of the relation table that maps to the fourth column
given. The fourth column specifies the column of your destination table to use for
mapping. In our example, the relation table "persons_addresses" has a column
"address_id", which is a foreign key referring to the column "id" in the table
"addresses".

As with ezcPersistentSingleTableMap instances, you can use multiple
mappings in one ezcPersistentManyToManyRelation->columnMap array. Here, we use
a person's first and last name for the mapping: ::

    <?php
    // ...

    $def->relations["Address"] = new ezcPersistentManyToManyRelation(
        "persons",
        "addresses",
        "persons_addresses"
    );
    $def->relations["Address"]->columnMap = array(
        new ezcPersistentDoubleTableMap( "firstname", "person_firstname", "address_id", "id" ),
        new ezcPersistentDoubleTableMap( "lastname", "person_lastname", "address_id", "id" )
    );

    return $def;
    ?>

Using n:m relations
-------------------

As stated earlier, the usage methods behave slightly differently when dealing
with n:m relations. If you use ezcPersistentSession->addRelatedObject(), the 
desired relation record is inserted into the relation table. The same applies to the
removeRelatedObject() method of ezcPersistentSession, which deletes the specific
record. This also means that you do not need to store the affected objects
explicitly after altering the relations between them. If you have made other
changes to the objects, they must be stored to save the changes.

Aside from that, the ezcPersistentSession->delete() method keeps track of the
relation records. If you delete a record, all of its relation records are
automatically deleted.

Special 1:1 relations
---------------------

If you want to use 1:1 relations, where two tables share a common primary key,
you need to define a table to generate the key and the other table to use
ezcPersistentManualGenerator. For our example, if one person may only have one
address, the definition would be as follows: ::

    <?php
    $def = new ezcPersistentObjectDefinition();
    $def->table = "addresses";
    $def->class = "Address";

    $def->idProperty                = new ezcPersistentObjectIdProperty;
    $def->idProperty->columnName    = 'id';
    $def->idProperty->propertyName  = 'id';
    $def->idProperty->generator     = new ezcPersistentGeneratorDefinition( 'ezcPersistentManualGenerator' );

    return $def;
    ?>

This is the relation (defined in the definition file of the Person): ::

    <?php
    // ...

    $def->relations["Address"] = new ezcPersistentOneToOneRelation(
        "persons",
        "addresses"
    );
    $def->relations["Address"]->columnMap = array(
        new ezcPersistentSingleTableMap(
            "id",
            "person_id"
        )
    );

    return $def;
    ?>

If you let both tables use ezcPersistentSequenceGenerator for the same key,
ezcPersistentSession will fail to save a related object, since the id will
already be set by the ezcPersistentSession::addRelatedObject() method.

Another way to make this work is to not use the same primary key for both
tables, but to make the Address object have its own id and only use the Person
id as a foreign key.

Reverse relations
-----------------

Since you can always look at a relation from two sides, ezcPersistentRelation
implementations can be configured to be "reverse". A reverse relation
indicates that the relation is already defined in the opposite direction and
that the original direction is the main used one. The one
marked as "reverse" is a secondary one, for consistency reasons. For a relation
that is marked as reverse, it is not possible to use
ezcPersistentSession->addRelatedObject() and
ezcPersistentSession->removeRelatedObject(). You can still use
ezcPersistentSession->getRelatedObjects() for relations that are flagged
"reverse".

For most relation types, the reverse attribute of the relation definition object
is set to false by default. You can manually set it. Exceptions are
ezcPersistentManyToOneRelation relations. This relation type only makes sense
as a reverse relation for ezcPersistentOneToManyRelation. Therefore, the
reverse attribute is set to false for ezcPersistentManyToOneRelation and is not
publicly accessible for writing.

The following example shows the reverse relation definition for the n:m relations
example::

    <?php
    // ...

    $def->relations["Person"] = new ezcPersistentManyToManyRelation(
        "addresses",
        "persons",
        "persons_addresses"
    );
    $def->relations["Address"]->columnMap = array(
        new ezcPersistentDoubleTableMap( "id", "address_id", "person_id", "id" ),
    );
    $def->relations["Address"]->reverse = true;

    return $def;
    ?>

With the relation definition shown above, you would still be able to relate the
Persons object to an Address object, but not to add or remove related
Person objects to/from an Address. In other words, the following code still
works: ::

    <?php
    $address = $session->load( "Address", 23 );
    $persons = $session->getRelatedObjects( $address, "Person" );
    // ...
    ?>

While the following would not work: ::

    <?php
    // ...
    foreach ( $persons as $person )
    {
        $session->removeRelatedObject( $address, $person );
    }
    ?>

Instead, only the other direction works, because this one is the main
direction. ::

    <?php
    // ...
    foreach ( $persons as $person )
    {
        $session->removeRelatedObject( $person, $address );
    }
    ?>

Cascading deletes
-----------------

Cascading relations are done through the flag "cascade" of a
ezcPersistentRelation implementation. All implementations except the
ezcPersistentManyToManyRelation class support this flag. It allows you to
automatically delete all related objects for a source object when the source
object is deleted.

The following example shows how to add a cascading relation to a relation
definition, based on the example from `Defining a simple relation`_::

    <?php
    // ...

    $def->relations["Address"] = new ezcPersistentOneToManyRelation(
        "persons",
        "addresses"
    );
    $def->relations["Address"]->columnMap = array(
        new ezcPersistentSingleTableMap(
            "id",
            "person_id"
        ),
    );
    $def->relations["Address"]->cascade = true;

    return $def;
    ?>

If you now use the following, the Person object and all related Address objects
are deleted: ::

    <?php
    $person = $session->load( "Person", 1 );
    $session->delete( $person );
    ?>

Beware that this does not work with ezcPersistentManyToManyRelation instances,
because it could cause serious inconsistencies in your data.



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