First idea for SQL Filtering in Doctrine2

example.php

<?php

class MySoftDeleteFilter extends SQLFilter
{
    public function addFilterConstraint(ClassMetadata $targetEntity, $targetTableAlias, Connection $conn)
    {
        if ($targetEntity->name != "MyEntity\SoftDeleteNewsItem") {
            return "";
        }
        
        return ' AND '.$targetTableAlias.'.deleted = 0';
    }
}

class MyLocaleFilter extends SQLFilter
{
    public function addFilterConstraint(ClassMetadata $targetEntity, $targetTableAlias)
    {
        if (!in_array("LocaleAware", $targetEntity->reflClass->getInterfaceNames())) {
            return "";
        }
        
        return ' AND ' . $targetTableAlias.'.locale = ' . $this->getParam('locale'); // getParam uses connection to quote the value.
    }
}

$config->addFilter("locale", "MyLocaleFilter");
$config->addFilter("soft_delete", "MySoftDeleteFilter");

$em = EntityManager::create($params, $config);

$em->enableFilter("locale")->setParameter("locale", "en");

SQLFilter.php

<?php

namespace Doctrine\ORM\Query\Filter;

abstract class SQLFilter
{
    final public function __construct(Connection $conn);
    
    final function setParameter($name, $value, $type);
    
    final function getParameter($name);

    abstract function addFilterConstraint(ClassMetadata $targetEntity, $targetTableAlias);
}

namespace Doctrine\ORM;

class Configuration
{
    public function addFilter($name, $className);

    public function getFilterClassName($name);
}

class EntityManager
{
    private $enabledFilters;
    /** @return SQLFilter[] */
    public function getEnabledFilters();
    /** Throws exception if filter does not exist. No-op if the filter is alrady enabled.
     * @return SQLFilter */
    public function enableFilter($name);
    /** Disable the filter, looses the state
     * @return SQLFilter */
    public function disableFilter($name) { unset($this->enabledFilters[$name]); }
    /** throws exception if not in enabled filters
     * @return SQLFilter */
    public function getFilter($name);
}

class Query
{
   // needs to serialize the enabled filters + their parameters for the DQL Hash to SQL Cache Computation.
}