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Saving a ManyToMany Relation + Joins

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We now know that a ManyToMany relationship works with the help of a join table. The question now is: how can we insert new records into that join table? How can I relate an Article to several tags?

The answer is the exactly the same as our ManyToOne relation. Start by opening BaseFixture. At the bottom, I'm going to paste in a new protected function called getRandomReferences(). We already have and getRandomReference() method that returns just one object. This is the same, but you can pass it a class name and how many of those objects you want back. The objects you get back may or may not be a unique set - the method isn't perfect, but, it's good enough.

Next, in ArticleFixtures, this is where we'll set the relationship. And that means, we need to make sure that TagFixture is loaded first so that can reference those tags here. At the top, add implements DependentFixtureInterface. Then, I'll go to the Code -> Generate menu - or Command + N on a Mac - select "Implement Methods" and choose getDependencies(). We now depend on TagFixture::class.

Above, let's first get some tag objects: $tags = $this->getRandomReferences() and pass it Tag::class, and then, let's fetch $this->faker->numberBetween() zero and five. So, find 0 to 5 random tags.

And just to make sure you that this does give us Tag objects, dump($tags) and die. Then find your terminal and run:

php bin/console doctrine:fixtures:load

Explaining Proxies

Perfect! These are Tag objects. Oh, by the way, sometimes you may notice that your entity's class name is prefixed with this weird Proxies stuff. When you see that, just ignore it. A "Proxy" is a special class that Doctrine generates and sometimes wraps around your real object. Doctrine does that so that it can do its relationship lazy-loading magic.

Actually, check this out: it looks like all the data on this Tag is null! But, that's not true. As soon as you reference some data on that Tag, Doctrine will query for the data and fill it in. That's lazy-loading in action.

Let me show you: add a foreach over $tags as $tag. To help PhpStorm, I'll add some inline PHPDoc to tell it that $tags is an array of Tag objects.

Inside the loop, just say $tag->getName(). I know, that looks weird: we're calling a method but not using it! But, calling this method should be enough to trigger Doctrine to go query for this tag's real data. Below, dump($tag) and die after the loop.

Load the fixtures again:

php bin/console doctrine:fixtures:load

Boom! We have data!

Anyways, this is the proxy system in action. You need to know what it is so because you will see it from time-to-time. But mostly, it should be completely invisible: don't think about it.

Adding Tags to Article

Finally, how can we add each Tag to the Article? No surprise, it's $article->addTag(). Try the fixtures again:

php bin/console doctrine:fixtures:load

Ok, no errors. To the database!

php bin/console doctrine:query:sql 'SELECT * FROM tag'

Yep, 10 tags with various, weird names. Now look for:

php bin/console doctrine:query:sql 'SELECT * FROM article_tag'

Let's see what the join table looks like. Yea! 24 rows! Each time we add a Tag to Article and save, Doctrine inserts a row in this table. If we were to remove an existing Tag from an Article object with $article->removeTag() and thne flush, Doctrine would actually delete that row. For the first, and only, time, we have a table that we don't need to think about, at all: Doctrine inserts and deletes data for us.

All we need to do is worry about relating Article objects to Tag objects. Doctrine takes care of the saving.

Rendering the Tags

And now we can turn back to building our site: open the article/show.html.twig template. On this page, let's print the tags right under the article title. So, scroll down a bit. Copy the span for the heart count and paste it below.

Because the Article object holds an array of tags, use for tag in article.tags. Inside, let's create a cute little badge and print the tag name: {{ tag.name }}.

That's super easy. Try it: find the page and refresh. We got it! Well, this Article only has one tag - boring. Find a different one: boom! Four different tags.

Let's repeat this on the homepage: we'll list the tags right under the title. Copy the for loop, then open homepage.html.twig. Down below, add a <br>, then paste! Wrap this in a <small> tag and change the class to badge-light.

So, this is just the same thing again: we have an article variable, so we can loop over its tags.

But notice, before we refresh, there are 8 queries. But after, there are 15! The page works, but we have another N+1 query problem. And, it's probably no big deal, but let's learn how to JOIN in a ManyToMany query so we can fix it.

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