Persisting a document suddenly stops returning next identifier value (ALNUM strategry) - mongodb

Question

I have Symfony 2.6 application and using doctrine-odm-bundle 3.0.2 (doctrine-odm: 1.1.2, mongodb: 1.4.0).
My document has referenceMany and referenceOne in attributes and when I create new instance of it, fill fields and persist - it goes fine in the begining. I can create few nearly empty documents, with referenced document(s) or without and it works fine. At some point I am trying to add new item in the database and getting an error:
E11000 duplicate key error collection: test.Product index: _id_ dup key: { : 0 }
The message is clear - I can see that there was a document added to the collection with id = 0, therefore second one can't go -> duplicate entry. But why it suddenly starts to return "0" for id? Even though, I checked doctrine_increment_ids collection - counter for id is being incremented. But $product->getId() becomes "0" after persist.
If I drop the database and start all over - it works, I can still add new products in the collection. Let's say I successfully created 12 products. Creating 13th resulting a document with id=0 being persisted in the collection. 14th fails with duplicate error.
Can you please help to troubleshoot or suggest an idea where does it go wrong?
P.S> I am not considering an upgrade of Symfony2 (at this point) neither as doctrine-odm-bundle (it depends on newer Symfony2 as well) - migration efforts are quite high and I am not sure it will fix the issue. First I want to find out the root cause.
// Document Product
/**
* #MongoDB\Document
* #MongoDB\HasLifecycleCallbacks
*/
class Product
{
/** #MongoDB\Id(strategy="ALNUM", type="int") */
protected $id;
/**
* #Gedmo\ReferenceOne(type="entity", class="Entity\User", inversedBy="products", identifier="userId")
*/
protected $user;
/**
* #MongoDB\Field(name="user_id", type="int")
*/
protected $userId;
/**
* #MongoDB\ReferenceMany(
* targetDocument="Picture",
* discriminatorMap={"file" = "File", "picture" = "Picture"},
* discriminatorField="discr",
* defaultDiscriminatorValue="picture"
* )
* #Assert\Valid
*/
protected $pictures;
...
}
// Entity User
/**
* User entity
* #ORM\Entity
* #ORM\Table(name="users")
*/
class User
{
/**
* #ORM\Id
* #ORM\Column(type="integer")
* #ORM\GeneratedValue(strategy="AUTO")
*/
protected $id;
/**
* #var ArrayCollection $textures
*
* #Gedmo\ReferenceMany(type="document", class="Document\Product", mappedBy="user")
*/
protected $products;
...
}
// Document Picture
/**
* #MongoDB\Document
* #MongoDB\InheritanceType("SINGLE_COLLECTION")
* #MongoDB\DiscriminatorField("discr")
* #MongoDB\DiscriminatorMap({"file" = "File", "picture" = "Picture"})
* #MongoDB\DefaultDiscriminatorValue("picture")
* #MongoDB\HasLifecycleCallbacks
*/
class Picture
{
/**
*
* #MongoDB\Id(strategy="ALNUM", type="int")
*/
protected $id;
/**
* #MongoDB\ReferenceOne(targetDocument="Product")
*
* #var Product $product
*/
protected $product;
...
}

Answer 1

Documentation reading always helps (generation strategies). Basically, strategy="ALNUM" and type="int" just can't go together :)
Change strategy to INCREMENT and remove type="int" if you want to have integers in your _id.
Or you can change type to string to continue with _id being an alphanumeric string.