您没有粘贴src/Blogger/BlogBundle/Entity/Blog.php文件。这将有助于解决你的问题。
最有可能的是你没有添加注解字段到你的实体中（或者没有正确地注解它）。
类似问题：Doctrine2: What is wrong with the association between these entities?
编辑：现在当你粘贴你的实体时，我可以看到注解字段没有注解。教义的实体管理员对此一无所知。您必须提供Map（在您的情况下通过注解）。
编辑2：
在您的实体中，您应该具有（src/Blogger/BlogBundle/Entity/Blog.php）：

/**
 * @ORM\OneToMany(targetEntity="Comment", mappedBy="blog")
 */
protected $comments;

但你有：

protected $comments;

缺少Map。教义不知道如何使用你的领域。

我也遵循了symblog教程。我的blog.php文件如下：

<?php

namespace Blogger\BlogBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Doctrine\Common\Collections\ArrayCollection;

/**
* Blogger\BlogBundle\Entity\Blog
*/
class Blog
{
    /**
    * @var integer $id
    */
    private $id;

    /**
    * Get id
    *
    * @return integer 
    */
  public function getId()
{
    return $this->id;
}
/**
 * @var string $title
 */
private $title;

/**
 * @var string $author
 */
private $author;

/**
 * @var string $blog
 */
private $blog;

/**
 * @var string $image
 */
private $image;

/**
 * @var string $tags
 */
private $tags;

/**
 * @var string $comment
 */
private $comment;

/**
 * @var datetime $created
 */
private $created;

/**
 * @var datetime $updated
 */
private $updated;

public function __construct()
{
    $this->comments = new ArrayCollection();
    $this->setCreated(new \DateTime());
    $this->setUpdated(new \DateTime());
}
/**
 * Set title
 *
 * @param string $title
 */
public function setTitle($title)
{
    $this->title = $title;
    $this->setSlug($this->title);
}

/**
 * Get title
 *
 * @return string 
 */
public function getTitle()
{
    return $this->title;
}

/**
 * Set author
 *
 * @param string $author
 */
public function setAuthor($author)
{
    $this->author = $author;
}

/**
 * Get author
 *
 * @return string 
 */
public function getAuthor()
{
    return $this->author;
}

/**
 * Set blog
 *
 * @param string $blog
 */
public function setBlog($blog)
{
    $this->blog = $blog;
}

/**
 * Get blog
 *
 * @return string 
 */
public function getBlog($length = null)
{
    if (false === is_null($length) && $length > 0)
        return substr($this->blog, 0, $length);
    else
        return $this->blog;
}

/**
 * Set image
 *
 * @param string $image
 */
public function setImage($image)
{
    $this->image = $image;
}

/**
 * Get image
 *
 * @return string 
 */
public function getImage()
{
    return $this->image;
}

/**
 * Set tags
 *
 * @param string $tags
 */
public function setTags($tags)
{
    $this->tags = $tags;
}

/**
 * Get tags
 *
 * @return string 
 */
public function getTags()
{
    return $this->tags;
}

/**
 * Set comment
 *
 * @param string $comment
 */
public function setComment($comment)
{
    $this->comment = $comment;
}

/**
 * Get comment
 *
 * @return string 
 */
public function getComment()
{
    return $this->comment;
}

/**
 * Set created
 *
 * @param datetime $created
 */
public function setCreated($created)
{
    $this->created = $created;
}

/**
 * Get created
 *
 * @return datetime 
 */
public function getCreated()
{
    return $this->created;
}

/**
 * Set updated
 *
 * @param datetime $updated
 */
public function setUpdated($updated)
{
    $this->updated = $updated;
}

/**
 * Get updated
 *
 * @return datetime 
 */
public function getUpdated()
{
    return $this->updated;
}

public function addComment(Comment $comment)
{
    $this->comments[] = $comment;
}

public function getComments()
{
    return $this->comments;
}

/**
 * @ORM\preUpdate
 */
public function setUpdatedValue()
{
     $this->setUpdated(new \DateTime());
}

/**
 * @var Blogger\BlogBundle\Entity\Comment
 */
private $comments;

public function __toString()
{
    return $this->getTitle();
}

/**
 * @var string $slug
 */
protected $slug;

/**
 * Set slug
 *
 * @param string $slug
 */
public function setSlug($slug)
{
    $this->slug = $this->slugify($slug);
}

/**
 * Get slug
 *
 * @return string 
 */
public function getSlug()
{
    return $this->slug;
}

public function slugify($text)
{
    $text = preg_replace('#[^\\pL\d]+#u', '-', $text);

    $text = trim($text, '-');

    if (function_exists('iconv'))
    {
        $text = iconv('utf-8', 'us-ascii//TRANSLIT', $text);
    }

    $text = strtolower($text);

    $text = preg_replace('#[^-\w]+#', '', $text);

    if (empty($text))
    {
        return 'n-a';
    }

    return $text;
  } }

希望这对你有帮助。
您的代码丢失

/**
* @var Blogger\BlogBundle\Entity\Comment
*/
private $comments;

也许你也遇到过同样的问题。
如果你知道你的关系和一切都很好。
我的解决方案：

Cache clear :)