How to use a many to many relationship in Symfony2

Question

I am working on a Symfony 2 based web app and struggling to create a many to many relationship between to entities:

Each Task entity should be assigned to any number of Categories . Of course each Category can be used by any number of Task entities. While a Task needs to know its Categories the Category class has no relationship back to the Tasks .

Following this tutorial I created:

class Task {    
    /**
    * @ORM\Column(name="categories")
    * @ORM\ManyToMany(targetEntity="MyBundle\Entity\Category")
    */
    protected $categories;

    public function __construct() {
        $this->productVariations = new \Doctrine\Common\Collections\ArrayCollection();
        ...
    }
}

Which results in the following SQL table:

CREATE TABLE task ... categories VARCHAR(255) NOT NULL, ...

A new Task is created use a form with the following type:

class TaskType extends AbstractType {
    public function buildForm(FormBuilderInterface $builder, array $options) {
        $builder
            ->add('categories', EntityType::class, array(
                'class' => 'MyBundle:Category',
                'choice_label' => 'name',
                'multiple' => true,
                'expanded' => true,
            ))
            ...
    }
}

Submitting the form and persiting the new Task instance result in a SQL row with the following content in the categories column:

Doctrine\Common\Collections\ArrayCollection@000000007448a17a0000000048941f2f

What does this mean? Storing the referenced Category entities as array is not normal form but of course valid (in fact I would prefer this solution over adding a relationship table since in practice the number of categories per task will be quite small, thus a third table would be more overhead).

However I would assume that the array stores the IDs of the referenced objects/rows in some form. How would it be possible to store any number of references in a VARCHAR(255) column?

While submitting and persisting creates a DB entry, the Category references are not stored correctly. When I try to re-read this entry, the categories property is empty.

Answer 1

You have redundant column definition. Try to simply remove following line:

* @ORM\Column(name="categories")

Then you need to update you database schema.

This line creates a varchar field in task table which shouldn't be there. Many to many relations are implemented via separate table.

If you want to store ids in a single field instead of separate table, then you will handle it on your own. That's because it won't be able to be a relation anymore. You loose possibility to create foreign key or doing any join queries. Also filtering records will be harder to achieve.

Answer 2

I think you're missing something here.

Like you said, a Task can be associated to many Category (ies) AND the Category class DOES NOT need a relational mapping to the Task class.

All you need in your Task class is a property to reference your Categories.

I think what you're looking for is this .