php - Symfony2 - Embedded forms wipe child data

Question

Yet another continuation of this and also this:

So, I'm able to initially show a form that contains a BlogPost and it's associated Comments. The problem is, if I edit some BlogPost info and save, it wipes out all of my Comments, regardless of whether or not they're related to that BlogPost. Not good.

Most of my relevant code is in the second link above, so I won't repeat it here. I will, however, add what's in my controller and view.

Controller:

public function EditBlogAction($id)
{
    $request = $this->get('request');
    $em = $this->get('doctrine')->getManager();
    $blogPost = $em->getRepository('Acme\SiteBundle\Entity\BlogPost')->find($id);

    $comments = $blogPost->getComments();

    $form = $this->createForm(new BlogPostType(), $blogPost);

    if ($request->getMethod() == 'POST') {
        $form->bind($request);

        foreach ($comments as $comment) {
            $em->persist($comment);
        }
        $em->persist($blogPost);

        $em->flush();
        $em->clear();
    }

    return $this->render('SiteBundle:Site:editblog.html.twig',array('blogpost' => $blogPost, 'form' => $form->createView()));
}

editblog.html.twig:

<form action='{{ path('_admin_blog_edit', { 'id':blogpost.getId }) }}' method='post' enctype="multipart/form-data">
    {{ form_widget(form.title) }}

    {# other blog post fields #}

    {% for comment in form.comments %}
        {{ comment.commentBody }}
    {% endfor %}
<input type="submit" />

I can't see why saving BlogPost info would wipe any Comments, let alone Comments not belonging to that particular post.