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Fetching Relations

Yes! Each Article is now related to two comments in the database. So, on the article show page, it's time to get rid of this hardcoded stuff and, finally, query for the true comments for this Article.

In src/Controller, open ArticleController and find the show() action:

... lines 1 - 16
class ArticleController extends AbstractController
{
... lines 19 - 40
/**
* @Route("/news/{slug}", name="article_show")
*/
public function show(Article $article, SlackClient $slack)
{
if ($article->getSlug() === 'khaaaaaan') {
$slack->sendMessage('Kahn', 'Ah, Kirk, my old friend...');
}
$comments = [
'I ate a normal rock once. It did NOT taste like bacon!',
'Woohoo! I\'m going on an all-asteroid diet!',
'I like bacon too! Buy some from my site! bakinsomebacon.com',
];
return $this->render('article/show.html.twig', [
'article' => $article,
'comments' => $comments,
]);
}
... lines 61 - 73
}

This renders a single article. So, how can we find all of the comments related to this article? Well, we already know one way to do this.

Remember: whenever you need to run a query, step one is to get that entity's repository. And, surprise! When we generated the Comment class, the make:entity command also gave us a new CommentRepository:

... lines 1 - 2
namespace App\Repository;
use App\Entity\Comment;
use Doctrine\Bundle\DoctrineBundle\Repository\ServiceEntityRepository;
use Symfony\Bridge\Doctrine\RegistryInterface;
/**
* @method Comment|null find($id, $lockMode = null, $lockVersion = null)
* @method Comment|null findOneBy(array $criteria, array $orderBy = null)
* @method Comment[] findAll()
* @method Comment[] findBy(array $criteria, array $orderBy = null, $limit = null, $offset = null)
*/
class CommentRepository extends ServiceEntityRepository
{
public function __construct(RegistryInterface $registry)
{
parent::__construct($registry, Comment::class);
}
... lines 21 - 49
}

Thanks MakerBundle!

Get the repository by adding a CommentRepository argument. Then, let's see, could we use one of the built-in methods? Try $comments = $commentRepository->findBy(), and pass this article set to the entire $article object:

... lines 1 - 6
use App\Repository\CommentRepository;
... lines 8 - 17
class ArticleController extends AbstractController
{
... lines 20 - 41
/**
* @Route("/news/{slug}", name="article_show")
*/
public function show(Article $article, SlackClient $slack, CommentRepository $commentRepository)
{
if ($article->getSlug() === 'khaaaaaan') {
... line 48
}
$comments = $commentRepository->findBy(['article' => $article]);
... lines 52 - 63
}
... lines 65 - 77
}

Dump these comments and die:

... lines 1 - 17
class ArticleController extends AbstractController
{
... lines 20 - 41
/**
* @Route("/news/{slug}", name="article_show")
*/
public function show(Article $article, SlackClient $slack, CommentRepository $commentRepository)
{
... lines 47 - 50
$comments = $commentRepository->findBy(['article' => $article]);
dump($comments);die;
... lines 53 - 63
}
... lines 65 - 77
}

Then, find your browser and, try it! Yes! It returns the two Comment objects related to this Article!

So, the weird thing is that, once again, you need to stop thinking about the columns in your tables, like article_id, and only think about the properties on your entity classes. That's why we use 'article' => $article. Of course, behind the scenes, Doctrine will make a query where article_id = the id from this Article. But, in PHP, we think all about objects.

Fetching Comments Directly from Article

As nice as this was... there is a much simpler way! When we generated the relationship, it asked us if we wanted to add an optional comments property to the Article class, for convenience. We said yes! And thanks to that, we can literally say $comments = $article->getComments(). Dump $comments again:

... lines 1 - 13
class ArticleController extends AbstractController
{
... lines 16 - 37
/**
* @Route("/news/{slug}", name="article_show")
*/
public function show(Article $article, SlackClient $slack)
{
... lines 43 - 46
$comments = $article->getComments();
dump($comments);die;
... lines 49 - 59
}
... lines 61 - 73
}

Oh, and now, we don't need the CommentRepository anymore:

... lines 1 - 13
class ArticleController extends AbstractController
{
... lines 16 - 37
/**
* @Route("/news/{slug}", name="article_show")
*/
public function show(Article $article, SlackClient $slack)
{
... lines 43 - 59
}
... lines 61 - 73
}

Cool.

Lazy Loading

Head back to your browser and, refresh! It's the exact same as before. Wait, what? What's this weird PersistentCollection thing?

Here's what's going on. When Symfony queries for the Article, it only fetches the Article data: it does not automatically fetch the related Comments. And, for performance, that's great! We may not even need the comment data! But, as soon as we call getComments() and start using that, Doctrine makes a query in the background to go get the comment data.

This is called "lazy loading": related data is not queried for until, and unless, we use it. To make this magic possible, Doctrine uses this PersistentCollection object. This is not something you need to think or worry about: this object looks and acts like an array.

To prove it, let's foreach over $comments as $comment and dump each $comment inside. Put a die at the end:

... lines 1 - 13
class ArticleController extends AbstractController
{
... lines 16 - 37
/**
* @Route("/news/{slug}", name="article_show")
*/
public function show(Article $article, SlackClient $slack)
{
... lines 43 - 46
$comments = $article->getComments();
foreach ($comments as $comment) {
dump($comment);
}
die;
... lines 52 - 62
}
... lines 64 - 76
}

Try it again! Boom! Two Comment objects!

Fetching the Comments in the Template

Back in the controller, we no longer need these hard-coded comments. In fact, we don't even need to pass comments into the template at all! That's because we can call the getComments() method directly from Twig!

Remove all of the comment logic:

... lines 1 - 13
class ArticleController extends AbstractController
{
... lines 16 - 37
/**
* @Route("/news/{slug}", name="article_show")
*/
public function show(Article $article, SlackClient $slack)
{
if ($article->getSlug() === 'khaaaaaan') {
$slack->sendMessage('Kahn', 'Ah, Kirk, my old friend...');
}
return $this->render('article/show.html.twig', [
'article' => $article,
]);
}
... lines 51 - 63
}

And then, jump into templates/article/show.html.twig. Scroll down a little... ah, yes! First, update the count: article.comments|length:

... lines 1 - 4
{% block body %}
<div class="container">
<div class="row">
<div class="col-sm-12">
<div class="show-article-container p-3 mt-4">
... lines 11 - 39
<div class="row">
<div class="col-sm-12">
<h3><i class="pr-3 fa fa-comment"></i>{{ article.comments|length }} Comments</h3>
... lines 43 - 71
</div>
</div>
</div>
</div>
</div>
</div>
{% endblock %}
... lines 80 - 86

Easy! Then, below, change the loop to use for comment in article.comments:

... lines 1 - 4
{% block body %}
<div class="container">
<div class="row">
<div class="col-sm-12">
<div class="show-article-container p-3 mt-4">
... lines 11 - 39
<div class="row">
<div class="col-sm-12">
<h3><i class="pr-3 fa fa-comment"></i>{{ article.comments|length }} Comments</h3>
... lines 43 - 57
{% for comment in article.comments %}
... lines 59 - 69
{% endfor %}
</div>
</div>
</div>
</div>
</div>
</div>
{% endblock %}
... lines 80 - 86

And because each comment has a dynamic author, print that with {{ comment.authorName }}. And the content is now comment.content:

... lines 1 - 4
{% block body %}
<div class="container">
<div class="row">
<div class="col-sm-12">
<div class="show-article-container p-3 mt-4">
... lines 11 - 39
<div class="row">
<div class="col-sm-12">
<h3><i class="pr-3 fa fa-comment"></i>{{ article.comments|length }} Comments</h3>
... lines 43 - 57
{% for comment in article.comments %}
<div class="row">
<div class="col-sm-12">
... line 61
<div class="comment-container d-inline-block pl-3 align-top">
<span class="commenter-name">{{ comment.authorName }}</span>
... lines 64 - 65
<span class="comment"> {{ comment.content }}</span>
... line 67
</div>
</div>
</div>
{% endfor %}
</div>
</div>
</div>
</div>
</div>
</div>
{% endblock %}
... lines 81 - 87

Oh, and, because each comment has a createdAt, let's print that too, with our trusty ago filter:

... lines 1 - 4
{% block body %}
<div class="container">
<div class="row">
<div class="col-sm-12">
<div class="show-article-container p-3 mt-4">
... lines 11 - 39
<div class="row">
<div class="col-sm-12">
<h3><i class="pr-3 fa fa-comment"></i>{{ article.comments|length }} Comments</h3>
... lines 43 - 57
{% for comment in article.comments %}
<div class="row">
<div class="col-sm-12">
... line 61
<div class="comment-container d-inline-block pl-3 align-top">
<span class="commenter-name">{{ comment.authorName }}</span>
<small>about {{ comment.createdAt|ago }}</small>
... line 65
<span class="comment"> {{ comment.content }}</span>
... line 67
</div>
</div>
</div>
{% endfor %}
</div>
</div>
</div>
</div>
</div>
</div>
{% endblock %}
... lines 81 - 87

Love it! Let's try it! Go back, refresh and... yes! Two comments, from about 17 minutes ago. And, check this out: on the web debug toolbar, you can see that there are two database queries. The first query selects the article data only. And the second selects all of the comment data where article_id matches this article's id - 112. This second query doesn't actually happen until we reference the comments from inside of Twig:

... lines 1 - 4
{% block body %}
<div class="container">
<div class="row">
<div class="col-sm-12">
<div class="show-article-container p-3 mt-4">
... lines 11 - 39
<div class="row">
<div class="col-sm-12">
<h3><i class="pr-3 fa fa-comment"></i>{{ article.comments|length }} Comments</h3>
... lines 43 - 57
{% for comment in article.comments %}
... lines 59 - 70
{% endfor %}
</div>
</div>
</div>
</div>
</div>
</div>
{% endblock %}
... lines 81 - 87

That laziness is a key feature of Doctrine relations.

Next, it's time to talk about the subtle, but super-important distinction between the owning and inverse sides of a relation.

Leave a comment!

  • 2018-09-10 Diego Aguiar

    Hey Mylan

    I believe your DB schema is out of sync, I mean, you added a new property to your entity but didn't update the schema

    php ./bin/console doctrine:schema:update --force

    pass --dump-sql first so you can see the SQL which will be executed

    Cheers!

  • 2018-09-10 Mylan

    Hello I get a error with:
    An exception has been thrown during the rendering of a template ("An exception occurred while executing 'SELECT t0.id AS id_1, t0.author_name AS author_name_2, t0.content AS content_3, t0.created_at AS created_at_4, t0.updated_at AS updated_at_5, t0.article_id AS article_id_6 FROM comment t0 WHERE t0.article_id = ?' with params [49]:

    SQLSTATE[42S22]: Column not found: 1054 Unknown column 't0.created_at' in 'field list'"). Please help me out

  • 2018-09-04 Victor Bocharsky

    Hey Dmitriy,

    Yeah, you nailed it! That's exactly the best way to check it. The "empty($comments)" always returns false because you check that $comments variable is empty, but that's not true, it's always an object. And thanks for sharing your solution with others!

    Cheers!

  • 2018-09-04 Дмитрий Ченгаев

    I find it's

    $article->getComments()->isEmpty()

  • 2018-09-04 Дмитрий Ченгаев

    How can I check that the PersistentCollection is empty?

    $comments = $article->getComments();

    empty($comments)

    Always return false.