SYMFONY2 LEFTJOIN query error

Question

There are two instances blog and comment:

• blog.comments stores the comment type
• comment.blog_id is the foreign key of blog

My query statement:

public function getLatestBlogs($limit=null){
    $qb = $this->createQueryBuilder('b')
        ->select('b','c')
        ->leftJoin('b.comments', 'c')
        ->addOrderBy('b.created', 'DESC')
    ;

    if (false === is_null($limit))
        $qb->setMaxResults($limit);

    return $qb->getQuery()->getResult(\Doctrine\ORM\Query::HYDRATE_OBJECT);
}

Error message:

Error: Call to a member function add() on a non-object in E:\xampp\htdocs\symfony\vendor\doctrine\orm\lib\Doctrine\ORM\PersistentCollection.php line 177
500 Internal Server Error - FatalErrorException 

Commment the main code, the others are generated

/**
 * Class Comment
 * @ORM\Entity(repositoryClass="Blogger\BlogBundle\Entity\CommentRepository")
 * @ORM\Table(name="comment")
 * @ORM\HasLifecycleCallbacks()
 */
class Comment{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    protected $id;

    /**
     * @ORM\Column(type="string",length=100)
     */
    protected $user;

    /**
     * @ORM\Column(type="text")
     */
    protected $comment;

    /**
     * @ORM\Column(type="boolean")
     */
    protected $approved;

    /**
     * @ORM\ManyToOne(targetEntity="Blog",inversedBy="comments")
     * @ORM\JoinColumn(name="blog_id",referencedColumnName="id")
     */
    protected $blog;

    /**
     * @ORM\Column(type="datetime")
     */
    protected $created;

    /**
     * @ORM\Column(type="datetime")
     */
    protected $updated;

 }

Blog main code:

class Blog{
/**
 * @ORM\Id
 * @ORM\Column(type="integer")
 * @ORM\GeneratedValue(strategy="AUTO")
 */
protected $id;

/**
 * @ORM\Column(type="string")
 */
protected $title;

/**
 * @ORM\Column(type="string",length=100)
 */
protected $author;

/**
 * @ORM\Column(type="text")
 */
protected $blog;

/**
 * @var
 * @ORM\Column(type="string",length=20)
 */
protected $image;

/**
 * @var
 * @ORM\Column(type="text")
 */
protected $tags;

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

/**
 * @var
 * @ORM\Column(type="datetime")
 */
protected $created;

/**
 * @var
 * @ORM\Column(type="datetime")
 */
protected $updated;

Answer 1

You have to post the code of your blog and comment categories, at least include the comments on the blog.

UPDATE1:

The default value of the comments attribute on your Blog class is an empty array, but Doctrine actually uses ArrayCollection. The add() method in your error message is provided by ArrayCollection.

use Doctrine\Common\Collections\ArrayCollection;

// 在Blog的constructor里给comments一个默认值：
public function __construct()
{
    $this->comments = new ArrayCollection();
}

Source code:

https://github.com/doctrine/collections/blob/master/lib/Doctrine/Common/Collections/ArrayCollection.php

UPDATE2:

Doctrine is an ORM, so it has an object-oriented interface both inside and outside.