Fetching a Relation's Data

Navigate to our homepage and click on any of the starships showcasing an 'In Progress' status. These are the ships from our fixtures.

You might notice that we're already listing the parts. But don't let that fool you: these parts are all hard-coded.

Now, how do we get the parts that are related to this ship?

Open the controller for this page: src/Controller/StarshipController.php

Say Hello to Auto-wiring

To query for parts, we typically by autowire the StarshipPartRepository. Start the same way here with a StarshipPartRepository $partRepository argument.

Next, set $parts, to $partRepository->findBy().

This is pretty standard stuff: if you want to query where some property equals a value, you use findBy() and pass the property name and the value. When it comes to relationships, it's the same darn thing. $parts = $partRepository->findBy(['starship' => $ship]).

And no, we're not doing Starship ID or anything of the sort. We're keeping IDs out of this. Instead, we pass the Starship object itself. going to pass the entire ship object. You could actually just pass the id if you're feeling lazy, but in the spirit of doctrine, relationships, and thinking about objects, passing the entire Starship object is the way to go.

Debugging and Celebrating

Let's debug and see what we've got. dd($parts).

Refresh, and voila! An array of 10 StarshipPart objects, all related to this Starship. Pretty awesome, right? If you think so, hold onto your pants.

Replace $parts = with ship->getParts(). Refresh! Instead of an array of StarshipPart objects, we get a PersistentCollection. object that looks... empty of an array of StarshipPart objects, we get a PersistentCollection. Remember the ArrayCollection that make:entity added to our Starship constructor? PersistentCollection and ArrayCollection are all part of the same collection family. They're objects but they act like arrays. Ok, but why does this collection look empty? That's because Doctrine is smart: It doesn't query for the parts until we need them. Loop over $ship->getParts() and dump $part

Suddenly that empty-looking collection is full of the 10 StarshipPart objects. Magic!

Lazy Relation Queries

There are two queries at play here. The first one is for the Starship, and the second one is for all the StarshipParts. The first comes from Symfony querying for the Starship based on the slug. The second happens the moment we foreach over the parts. At that moment, Doctrine says:

I just remembered: I don't actually have the StarshipParts data for this Starship. Let me go and get that for you.

Isn't that just amazing? Makes me want to throw a party for Doctrine.

Tidying Up and Looping Over Parts

Get rid of the parts variable entirely... and remove StarshipPartRepository: that was way too much work. Instead, set a parts variable to $ship->getParts().

Now that we've got our shiny new parts variable, Loop over that in the template. Open up templates/starship/show.html.twig and replace the hard-coded part with our loop: for part in parts %, part.name, part.price, part.notes, endfor

Still too much Work?

And we've done it! We've managed to display all 10 of our related parts, without doing any serious work thanks to $ship->getParts().

But you know what? Even this is too much work. Get rid of the parts variable entirely: for part in ship.parts.

And... still not broken! For kicks and giggles, let's also display the number of for this ship. ship.parts|length

We still have two queries, but Doctrine is smart. It knows we've already queried for all the StarshipParts, so when we count them, we don't need to make another count query.

Next up: we'll talk about an often-misunderstood topic in Doctrine relations: the owning vs inverse side of each relationship.