I am facing a small issue on Doctrine 2 that causes OneToOne relation needed to be part of the parent class. The error occurs when doing $content->getVersions() but not when doing $em->find("Model\Lantern\Content\Version", $id). The error message is:

Warning: Undefined index: summary in ORM/UnitOfWork.php:2012
Fatal error: Call to a member function setValue() on a non-object in ORM/UnitOfWork.php on line 2013

Error replication:

Simply by having a look at the model we can see what's causing the Bug, will avoid class annotations to make it simpler:

class Content{
    /**
    * @OneToMany(targetEntity="Version", mappedBy="content")
    */
    private $versions;
...
}

class Version{
    /**
     * @ManyToOne(targetEntity="Content", inversedBy="versions")
     * @JoinColumn(name="content_id", referencedColumnName="content_id")
     */
    private $content;
...
}

class Article extends Version{
    /** @OneToOne(targetEntity="Model\Lantern\Content\Fieldset\Summary", mappedBy="version", cascade={"persist", "delete"}) */
    private $summary;  
...
}

class Summary{
    /**
     * @OneToOne(targetEntity="Model\Lantern\Content\Version", inversedBy="summary")
     * @JoinColumn(name="version_id", referencedColumnName="version_id")
     */
    private $version;    
...
}

As you can see, Article::$summary targets Summary::$version, but Summary::$version targets Version::$summary. This in OOP is valid as Version is contained inside Article, so doing a downcasting will get Article::summary.

The reason I believe why using $em->find works is that $em->find already knows which child class is before doing map, and relations works fine.
On the other hand, when doing $content->getVersions(), Content::$versions targets Version, and looks like Doctrine does not resolve which child class it before mapping OneToOne relationshipg.

This bug doesn't occur with OneToMany or ManyToMany relationships.