学说ORM：YAML在哪里定义多对多关系？

Question

I'm using Silex with Doctrine ORM trying to create mappings to the database with YAML. I have a many to many relationship between the categories and products tables and wish to have categories on the owner side. However, whenever I try to dump '$productCategories' in the twig file by

{% for productCategories in category.productCategories %}
<li><a href=""></a>{{ productCategories.name }}</li>
{% endfor %}

I receive:

Variable "productCategories" does not exist in "category.twig"

Here is the Categories.yml file:

MVCStore\Entity\Category:
type: entity
table: categories
manyToMany:
    productCategories:
        targetEntity: MVCStore\Entity\Product
        inversedBy: category
        joinTable:
            name: products_categories
            joinColumns:
                products:
                    referencedColumnName: id
            inverseJoinColumns:
                categories:
                    referencedColumnName: id
id:
    id:
        type: integer
        generator:
            strategy: auto
fields:
    name:
        type: string
    parent_id:
        type: integer

And here is the products.yml file:

MVCStore\Entity\Product:
type: entity
table: products
repositoryClass: MVCStore\EntityRepository\Product
oneToMany:
    productImages:
        targetEntity: MVCStore\Entity\ProductImage
        mappedBy: product
manyToMany:
    productCategories:
        targetEntity: MVCStore\Entity\Category
        mappedBy: products
id:
    id:
        type: integer
        generator:
            strategy: auto
fields:
    stock:
        type: integer
    name:
        type: string
    description:
        type: string
    featured:
        type: boolean
    price:
        type: decimal

If anything more is needed please ask.