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Param Converter & 404's


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We've programmed the happy path. When I go to /mix/13, my database does find a mix with that id and... life is good. But what if I change this to /99? Yikes. That's a 500 error: not something we want our site to ever do. This really should be a 404 error. So, how do we trigger a 404?

Triggering a 404 Page

Over in the method, this $mix variable will either be a VinylMix object or null if one isn't found. So we can say if (!$mix), and then, to trigger a 404, throw $this->createNotFoundException(). You can give this a message if you want, but it'll only be seen by developers.

49 lines | src/Controller/MixController.php
// ... lines 1 - 11
class MixController extends AbstractController
// ... lines 14 - 35
public function show($id, VinylMixRepository $mixRepository): Response
// ... lines 38 - 39
if (!$mix) {
throw $this->createNotFoundException('Mix not found');
// ... lines 43 - 46

This createNotFoundException(), as the name suggests, creates an exception object. So we're actually throwing an exception here... which is nice, because it means that code after this won't be executed.

Now, normally if you or something in your code throws an exception, it will trigger a 500 error. But this method creates a special type of exception that maps to a 404. Watch! Over here, in the upper right, when I refresh... 404!

By the way, this is not what the 404 or 500 pages would look like on production. If we switched to the prod environment, we'd see a pretty generic error page with no details. Then you customize how those look, even making separate styles for 404 errors, 403 Access Denied errors, or even... gasp ... 500 errors if something goes really wrong. Check out the Symfony docs for how to customize error pages.

Param Converter: Automatic Query

Okay! We've queried for a single VinylMix object and even handled the 404 path. But we can do this with way less work. Check it out! Replace the $id argument with a new argument, type-hinted with our entity class VinylMix. Call it, how about, $mix to match the variable below. Then... delete the query... and also the 404. And now, we don't even need the $mixRepository argument at all.

43 lines | src/Controller/MixController.php
// ... lines 1 - 35
public function show(VinylMix $mix): Response
return $this->render('mix/show.html.twig', [
'mix' => $mix,
// ... lines 42 - 43

This... deserves some explanation. So far, the "things" that we are "allowed" to have as arguments to our controllers are (1) route wildcards like $id or (2) services. Now we have a third thing. When you type-hint an entity class, Symfony will query for the object automatically. Because we have have a wildcard called {id}, it will take this value (so "99" or "16") and query for a VinylMix whose id is equal to that. The name of the wildcard - id in this case - needs to match the property name it should use for the query.

But if I go back and refresh... it doesn't work!?

Cannot autowire argument $mix of MixController::show(): it references VinylMix but no such service exists.

We know this isn't a service... so that make sense. But... why isn't it querying for the object like I just said it would?

Because... to get this feature to work, we need to install another bundle! Well, if you're using Symfony 6.2 and a new enough DoctrineBundle - probably version 2.8 - then this should work without needing anything else. But since we're using Symfony 6.1, we need one extra library.

Find your terminal and say:

composer require sensio/framework-extra-bundle

This is a bundle full of nice little shortcuts that, by Symfony 6.2, will all have been moved into Symfony itself. So eventually, you won't need this.

And now... without doing anything else... it works! It automatically queried for the VinylMix object and the page renders! And if you go to a bad ID, like /99... yes! Check it out! We get a 404! This feature is called a "ParamConverter"... which is mentioned in the error:

VinylMix object not found by the @ParamConverter annotation.

Anyways, I love this feature. If I need to query for multiple objects, like in the browse() action, I'll use the correct repository service. But if I need to query for a single object in a controller, I use this trick.

Next, let's make it possible to up vote and down vote our mixes by leveraging a simple form. To do this, for the first time, we will update an entity in the database.