What is the role played by the `type` column in Doctrine class table inheritance?

manpreet Best Answer 3 years ago

Using Symfony 4 / Doctrine 2.6. I have two entities Post and Comment. I want both to be taggable. So I create an entity Tag. I use Doctrine's class table inheritance to create the relationship:

/**
 * @ORM\Entity(repositoryClass="App\Repository\TagRepository")
 * @ORM\InheritanceType("JOINED")
 * @ORM\DiscriminatorColumn(name="type", type="string")
 * @ORM\DiscriminatorMap({"post" = "PostTag", "comment" = "CommentTag"})
 */
abstract class Tag
{
    /**
     * @ORM\Id()
     * @ORM\GeneratedValue()
     * @ORM\Column(type="integer")
     */
    private $id;

    /**
     * @ORM\Column(type="string", length=255)
     */
    private $title;

    // Getters and setters...
}

/** @ORM\Entity */
class PostTag extends Tag
{
    /**
     * @ORM\ManyToOne(targetEntity="App\Entity\Post", inversedBy="tags")
     */
    private $post;

    public function getPost(): ?Post
    {
        return $this->post;
    }
}

/** @ORM\Entity */
class CommentTag extends Tag
{
    /**
     * @ORM\ManyToOne(targetEntity="App\Entity\Comment", inversedBy="comments")
     */
    private $comment;

    public function getComment(): ?Comment
    {
        return $this->comment;
    }
}

This creates 3 tables: tag, post_tag, and comment_tag. The post_tag table structure looks like this:

id | post_id

The tag table structure looks like this:

id | title | type

How are e.g. posts and tags associated? If I want to associate post 13 with tag test, would the result be this:

post_tag table:

id | post_id
------------
1  | 13

tag table:

id | title | type
-----------------
1  | test  | post

?

If so, then what if I want to associate the same tag (test) with a comment. Would the tag table then look like this?

id | title | type
--------------------
1  | test  | post
2  | test  | comment

That seems a bit redundant. The same entity (the test tag) is then represented by 2 rows in the tag table. Am I getting this wrong?