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ManyToMany: The Inverse Side of the Relationship

Our goal is clear: list all of the genuses studied by this User.

The Owning vs Inverse Side of a Relation

Back in our Doctrine Relations tutorial, we learned that every relationship has two different sides: a mapping, or owning side, and an inverse side. In that course, we added a GenusNote entity and gave it a ManyToOne relationship to Genus:

101 lines src/AppBundle/Entity/GenusNote.php
... lines 1 - 10
class GenusNote
{
... lines 13 - 39
/**
* @ORM\ManyToOne(targetEntity="Genus", inversedBy="notes")
* @ORM\JoinColumn(nullable=false)
*/
private $genus;
... lines 45 - 99
}

This is the owning side, and it's the only one that we actually needed to create.

If you look in Genus, we also mapped the other side of this relationship: a OneToMany back to GenusNote:

189 lines src/AppBundle/Entity/Genus.php
... lines 1 - 14
class Genus
{
... lines 17 - 65
/**
* @ORM\OneToMany(targetEntity="GenusNote", mappedBy="genus")
* @ORM\OrderBy({"createdAt" = "DESC"})
*/
private $notes;
... lines 71 - 187
}

This is the inverse side of the relationship, and it's optional. When we mapped the inverse side, it caused no changes to our database structure. We added it purely for convenience, because we decided it sure would be fancy and nice if we could say $genus->getNotes() to automagically fetch all the GenusNotes for this Genus.

With a ManyToOne relationship, we don't choose which side is which: the ManyToOne side is always the required, owning side. And that makes sense, it's the table that holds the foreign key column, i.e. GenusNote has a genus_id column.

Owning and Inverse in ManyToMany

We can also look at our ManyToMany relationship in two different directions. If I have a Genus object, I can say:

Hello fine sir: please give me all Users related to this Genus.

But if I have a User object, I should also be able to say the opposite:

Good evening madame: I would like all Genuses related to this User.

The tricky thing about a ManyToMany relationship is that you get to choose which side is the owning side and which is the inverse side. And, I hate choices! The choice does have consequences.... but don't worry about that - we'll learn why soon.

Mapping the Inverse Side

Since we only have one side of the relationship mapped now, it's the owning side. To map the inverse side, open User and add a new property: $studiedGenuses. This will also be a ManyToMany with targetEntity set to Genus. But also add mappedBy="genusScientists:

223 lines src/AppBundle/Entity/User.php
... lines 1 - 16
class User implements UserInterface
{
... lines 19 - 77
/**
* @ORM\ManyToMany(targetEntity="Genus", mappedBy="genusScientists")
*/
private $studiedGenuses;
... lines 82 - 221
}

That refers to the property inside of Genus:

192 lines src/AppBundle/Entity/Genus.php
... lines 1 - 14
class Genus
{
... lines 17 - 71
/**
* @ORM\ManyToMany(targetEntity="User")
* @ORM\JoinTable(name="genus_scientist")
*/
private $genusScientists;
... lines 77 - 190
}

Now, on that property, add inversedBy="studiedGenuses, which points back to the property we just added in User:

192 lines src/AppBundle/Entity/Genus.php
... lines 1 - 14
class Genus
{
... lines 17 - 71
/**
* @ORM\ManyToMany(targetEntity="User", inversedBy="studiedGenuses")
* @ORM\JoinTable(name="genus_scientist")
*/
private $genusScientists;
... lines 77 - 190
}

When you map both sides of a ManyToMany relationship, this mappedBy and inversedBy configuration is how you tell Doctrine which side is which. We don't really know why that's important yet, but we will soon.

Back in User, remember that whenever you have a relationship that holds a collection of objects, like a collection of "studied genuses", you need to add a __construct function and initialize that to a new ArrayCollection():

223 lines src/AppBundle/Entity/User.php
... lines 1 - 4
use Doctrine\Common\Collections\ArrayCollection;
... lines 6 - 16
class User implements UserInterface
{
... lines 19 - 82
public function __construct()
{
$this->studiedGenuses = new ArrayCollection();
}
... lines 87 - 221
}

Finally, since we'll want to be able to access these studiedGenuses, go to the bottom of User and add a new public function getStudiedGenuses(). Return that property inside. And of course, we love PHP doc, so add @return ArrayCollection|Genus[]:

223 lines src/AppBundle/Entity/User.php
... lines 1 - 4
use Doctrine\Common\Collections\ArrayCollection;
... lines 6 - 16
class User implements UserInterface
{
... lines 19 - 214
/**
* @return ArrayCollection|Genus[]
*/
public function getStudiedGenuses()
{
return $this->studiedGenuses;
}
}

Using the Inverse Side

And just by adding this new property, we are - as I love to say - dangerous.

Head into the user/show.html.twig template that renders the page we're looking at right now. Add a column on the right side of the page, a little "Genuses Studied" header, then a ul. To loop over all of the genuses that this user is studying, just say for genusStudied in user.studiedGenuses. Don't forget the endfor:

56 lines app/Resources/views/user/show.html.twig
... lines 1 - 2
{% block body %}
<div class="container">
<div class="row">
... lines 6 - 38
<div class="col-xs-4">
<h3>Genus Studied</h3>
<ul class="list-group">
{% for genusStudied in user.studiedGenuses %}
... lines 43 - 49
{% endfor %}
</ul>
</div>
</div>
</div>
{% endblock %}

Inside, add our favorite list-group-item and then a link. Link this back to the genus_show route, passing slug set to genusStudied.slug. Print out genusStudied.name:

56 lines app/Resources/views/user/show.html.twig
... lines 1 - 2
{% block body %}
<div class="container">
<div class="row">
... lines 6 - 38
<div class="col-xs-4">
<h3>Genus Studied</h3>
<ul class="list-group">
{% for genusStudied in user.studiedGenuses %}
<li class="list-group-item">
<a href="{{ path('genus_show', {
'slug': genusStudied.slug
}) }}">
{{ genusStudied.name }}
</a>
</li>
{% endfor %}
</ul>
</div>
</div>
</div>
{% endblock %}

But will it blend? I mean, will it work? Refresh!

Hot diggity dog! There are the three genuses that this User studies. We did nothing to deserve this nice treatment: Doctrine is doing all of the query work for us.

In fact, click the database icon on the web debug toolbar to see what the query looks like. When we access the property, Doctrine does a SELECT from genus with an INNER JOIN to genus_scientist where genus_scientist.user_id equals this User's id: 11. That's perfect! Thanks Obama!

Ordering the Collection

The only bummer is that we can't control the order of the genuses. What if we want to list them alphabetically? We can't - we would instead need to make a custom query for the genuses in the controller, and pass them into the template.

What? Just kidding! In User, add another annotation: @ORM\OrderBy({"name" = "ASC"):

224 lines src/AppBundle/Entity/User.php
... lines 1 - 16
class User implements UserInterface
{
... lines 19 - 77
/**
* @ORM\ManyToMany(targetEntity="Genus", mappedBy="genusScientists")
* @ORM\OrderBy({"name" = "ASC"})
*/
private $studiedGenuses;
... lines 83 - 222
}

Refresh that!

If you didn't see a difference, you can double-check the query to prove it. Boom! There's our new ORDER BY. Later, I'll show you how you can mess with the query made for collections even more via Doctrine Criteria.

But up next, the last missing link: what if a User stops studying a Genus? How can we remove that link?

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