Doctrine 2 and Many-to-many link table with an extra field

Question

(Sorry for my incoherent question: I tried to answer some questions as I was writing this post, but here it is:)

I'm trying to create a database model with a many-to-many relationship inside a link table, but which also has a value per link, in this case a stock-keeping table. (this is a basic example for more problems I'm having, but I thought I'd just test it with this before I would continue).

I've used exportmwb to generate the two Entities Store and Product for this simple example, both are displayed below.

However, the problem now is that I can't figure out how to access the stock.amount value (signed int, as it can be negative) using Doctrine. Also, when I try to create the tables using doctrine's orm:schema-tool:create function

This yielded only two Entities and three tables, one as a link table without values and two data tables, as many-to-many relationships aren't entities themselves so I can only have Product and Store as an entity.

So, logically, I tried changing my database model to have stock as a separate table with relationships to store and product. I also rewrote the fieldnames just to be able to exclude that as a source of the problem:

Then what I found was that I still didn't get a Stock entity... and the database itself didn't have an 'amount'-field.

I really needed to be able to bind these stores and products together in a stock table (among other things)... so just adding the stock on the product itself isn't an option.

root@hdev:/var/www/test/library# php doctrine.php orm:info
Found 2 mapped entities:
[OK]   Entity\Product
[OK]   Entity\Store

And when I create the database, it still doesn't give me the right fields in the stock table:

So, looking up some things here, I found out that many-to-many connections aren't entities and thus cannot have values. So I tried changing it to a separate table with relationships to the others, but it still didn't work.

What am I doing wrong here?

Answer 1

A Many-To-Many association with additional values is not a Many-To-Many, but is indeed a new entity, since it now has an identifier (the two relations to the connected entities) and values.

That's also the reason why Many-To-Many associations are so rare: you tend to store additional properties in them, such as sorting , amount , etc.

What you probably need is something like following (I made both relations bidirectional, consider making at least one of them uni-directional):

Product:

namespace Entity;

use Doctrine\ORM\Mapping as ORM;

/** @ORM\Table(name="product") @ORM\Entity() */
class Product
{
    /** @ORM\Id() @ORM\Column(type="integer") */
    protected $id;

    /** ORM\Column(name="product_name", type="string", length=50, nullable=false) */
    protected $name;

    /** @ORM\OneToMany(targetEntity="Entity\Stock", mappedBy="product") */
    protected $stockProducts;
}

Store:

namespace Entity;

use Doctrine\ORM\Mapping as ORM;

/** @ORM\Table(name="store") @ORM\Entity() */
class Store
{
    /** @ORM\Id() @ORM\Column(type="integer") */
    protected $id;

    /** ORM\Column(name="store_name", type="string", length=50, nullable=false) */
    protected $name;

    /** @ORM\OneToMany(targetEntity="Entity\Stock", mappedBy="store") */
    protected $stockProducts;
}

Stock:

namespace Entity;

use Doctrine\ORM\Mapping as ORM;

/** @ORM\Table(name="stock") @ORM\Entity() */
class Stock
{
    /** ORM\Column(type="integer") */
    protected $amount;

    /** 
     * @ORM\Id()
     * @ORM\ManyToOne(targetEntity="Entity\Store", inversedBy="stockProducts") 
     * @ORM\JoinColumn(name="store_id", referencedColumnName="id", nullable=false) 
     */
    protected $store;

    /** 
     * @ORM\Id()
     * @ORM\ManyToOne(targetEntity="Entity\Product", inversedBy="stockProducts") 
     * @ORM\JoinColumn(name="product_id", referencedColumnName="id", nullable=false) 
     */
    protected $product;
}

Answer 2

Doctrine handles many-to-many relationships just fine.

The problem that you're having is that you don't need a simple ManyToMany association, because associations can't have "extra" data.

Your middle (stock) table, since it contains more than product_id and store_id, needs its own entity to model that extra data.

So you really want three classes of entity:

• Product
• StockLevel
• Store

and two associations:

• Product oneToMany StockLevel
• Store oneToMany StockLevel