gistfile1.php

<?php

/** @Document @HasLifecycleCallbacks */
class Product
{
    /** @Id */
    public $id;

    /** @String */
    public $name;

    /** @String */
    public $supplierName;

    /** @PrePersist @PreUpdate */
    public function cacheSupplierName()
    {
        $this->supplierName = $this->supplier->getName();
    }
}

/** @Document */
class Supplier
{
    /** @Id */
    public $id;

    /** @String */
    public $name;

    public function getName()
    {
        return $this->name;
    }
}