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The King of Relations: ManyToOne

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Selecting between ManyToOne and ManyToMany

Each genus will have many genus_notes. But, each genus_note that someone adds will relate to only one genus. There are only two possible types of relationships, and this is by far the most common. It's called a ManyToOne association.

The second type is called a ManyToMany association. To use a different example, this would be if each product had many tags, but also each tag related to many products.

And when it comes to Doctrine relations - don't trust the Internet! Some people will try to confuse you with other relationships like OneToMany, OneToOne and some garbage about unidirectional and bidirectional associations. Gross. Ignore it all. I guarantee, all of that will make sense really soon.

So your first job is simple: decide if you have a ManyToOne or ManyToMany relationship. And it's easy. Just answer this question:

Do either of the sides of the relationship belong to only one of the other?

Each genus_note belongs to only one genus, so we have a classic ManyToOne relationship.

Setting up a ManyToOne Relation

Forget about Doctrine: just think about the database. If every genus_note should belong to exactly one genus, How would you set that up? You'd probably add a genus_id column to the genus_note table. Simple!

Since we need to add a new column to GenusNote, open that entity class. You probably feel like you want to add a $genusId integer property here. That makes sense. But don't! Instead, add a $genus property and give it a ManyToOne annotation. Inside that, add targetEntity="Genus":

100 lines | src/AppBundle/Entity/GenusNote.php
// ... lines 1 - 10
class GenusNote
// ... lines 13 - 39
* @ORM\ManyToOne(targetEntity="Genus")
private $genus;
// ... lines 44 - 98


You can also use the full namespace: AppBundle\Entity\Genus - and that's required if the two entities do not live in the same namespace/directory.

Umm, guys? That's it. Relationship finished. Seriously.