doctrine 2: inheritance mapping join with parent table - php

Question

I have a base table like this:
class BaseProduct
{
/**
* #ORM\ManyToOne(targetEntity="ProductBundle\Entity\Category", inversedBy="baseProducts")
* #ORM\JoinColumn(name="menu_category_id", referencedColumnName="id", nullable=true)
**/
protected $category;
// ...
and another entity that inherit from BaseProduct
class ChildProduct extends BaseProduct
{
/**
* #ORM\ManyToOne(targetEntity="ProductBundle\Entity\Category", inversedBy="childProducts")
* #ORM\JoinColumn(name="menu_category_id", referencedColumnName="id", nullable=true)
**/
protected $category;
and Category entity:
class Category
{
/**
* #ORM\OneToMany(targetEntity="ProductBundle\Entity\BaseProduct", mappedBy="category")
* #ORM\OrderBy({"position"= "ASC"})
*/
private $baseProducts;
/**
* #ORM\OneToMany(targetEntity="ProductBundle\Entity\ChildProduct", mappedBy="category")
* #ORM\OrderBy({"position"= "ASC"})
*/
private $childProducts;
My ChildProduct table has one column named id and referenced to BaseProduct id's. Now I want to join Category with ChildProduct with this query:
$qb->select('mc', 'cp')
->from('ProductBundle:Category', 'mc')
->leftJoin('mc.childProducts', 'cp')
// .....
when I execute this query it gives this error:
ContextErrorException in SqlWalker.php line 922:
Notice: Undefined index: childProducts
While I have childProducts in Category.
Now I have two questions:
am I able to query on a parent field that does not exists in child table.
what's wrong with my query

Answer 1

Check doctrine chapter Inheritance Mapping: Inheritance Mapping
Following the documentation try this (I didn't test the code)
/** #ORM\MappedSuperclass */
class BaseProduct
{
/**
* #ORM\ManyToOne(targetEntity="ProductBundle\Entity\Category", inversedBy="baseProducts")
* #ORM\JoinColumn(name="menu_category_id", referencedColumnName="id", nullable=true)
**/
protected $category;
Now Doctrine knows that your class is a base class for others.
All you have to do it extend base class and add annotation #ORM\Entity
/**
* #ORM\Entity()
**/
class ChildProduct extends BaseProduct
{
/**
* #ORM\ManyToOne(targetEntity="ProductBundle\Entity\Category", inversedBy="childProducts")
* #ORM\JoinColumn(name="menu_category_id", referencedColumnName="id", nullable=true)
**/
protected $category;
I think you can remove from ChildProduct $category field. It should work also.