Keeping Your Modules Independent

One of the goals of using modules is to create discrete units of functionality that do not have many (if any) dependencies, allowing you to use that functionality in other applications without including unnecessary items.

Doctrine MongoDB ODM includes a utility called ResolveTargetDocumentListener, that functions by intercepting certain calls inside Doctrine and rewriting targetDocument parameters in your metadata mapping at runtime. This allows your bundle to use an interface or abstract class in its mappings while still allowing the mapping to resolve to a concrete document class at runtime.

This functionality allows you to define relationships between different documents without creating hard dependencies.

Background

In the following example, we have an InvoiceModule that provides invoicing functionality, and a CustomerModule that contains customer management tools. We want to keep these separated, because they can be used in other systems without each other; however, we’d like to use them together in our application.

In this case, we have an Invoice document with a relationship to a non-existent object, an InvoiceSubjectInterface. The goal is to get the ResolveTargetDocumentListener to replace any mention of the interface with a real class that implements that interface.

Configuration

We’re going to use the following basic documents (which are incomplete for brevity) to explain how to set up and use the ResolveTargetDocumentListener.

A Customer document:

<?php
// src/Acme/AppModule/Document/Customer.php

namespace Acme\AppModule\Document;

use Doctrine\ODM\MongoDB\Mapping\Annotations as ODM;
use Acme\CustomerModule\Document\Customer as BaseCustomer;
use Acme\InvoiceModule\Model\InvoiceSubjectInterface;

/**
 * @ODM\Document
 */
class Customer extends BaseCustomer implements InvoiceSubjectInterface
{
    // In our example, any methods defined in the InvoiceSubjectInterface
    // are already implemented in the BaseCustomer
}

An Invoice document:

<?php
// src/Acme/InvoiceModule/Document/Invoice.php

namespace Acme\InvoiceModule\Document;

use Doctrine\ODM\MongoDB\Mapping\Annotations as ODM;
use Acme\InvoiceModule\Model\InvoiceSubjectInterface;

/**
 * @ODM\Document
 */
class Invoice
{
    /**
     * @ODM\ReferenceOne(targetDocument="Acme\InvoiceModule\Model\InvoiceSubjectInterface")
     * @var InvoiceSubjectInterface
     */
    protected $subject;
}

An InvoiceSubjectInterface:

<?php
// src/Acme/InvoiceModule/Model/InvoiceSubjectInterface.php

namespace Acme\InvoiceModule\Model;

/**
 * An interface that the invoice Subject object should implement.
 * In most circumstances, only a single object should implement
 * this interface as the ResolveTargetDocumentListener can only
 * change the target to a single object.
 */
interface InvoiceSubjectInterface
{
    // List any additional methods that your InvoiceModule
    // will need to access on the subject so that you can
    // be sure that you have access to those methods.

    /**
     * @return string
     */
    public function getName();
}

Next, we need to configure the listener. Add this to the area where you setup Doctrine MongoDB ODM. You must set this up in the way outlined below, otherwise you cannot be guaranteed that the targetDocument resolution will occur reliably:

<?php
$evm  = new \Doctrine\Common\EventManager;
$rtdl = new \Doctrine\ODM\MongoDB\Tools\ResolveTargetDocumentListener;

// Adds a target-document class
$rtdl->addResolveTargetDocument(
    'Acme\\InvoiceModule\\Model\\InvoiceSubjectInterface',
    'Acme\\CustomerModule\\Document\\Customer',
    array()
);

// Add the ResolveTargetDocumentListener
$evm->addEventListener(\Doctrine\ODM\MongoDB\Events::loadClassMetadata, $rtdl);

// Create the document manager as you normally would
$dm = \Doctrine\ODM\MongoDB\DocumentManager::create($connectionOptions, $config, $evm);

Final Thoughts

With ResolveTargetDocumentListener, we are able to decouple our bundles so that they are usable by themselves and easier to maintain independently, while still being able to define relationships between different objects.

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