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09.

Relation OrderBy & fetch=EXTRA_LAZY

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You know what? On this page, we're missing the "created at" for each answer: I want to be able to see when each answer was posted. Let's fix that.

Head over to the template - show.html.twig - and, down here... right before the vote arrows, add a <small> tag and then {{ answer.createdAt }}. Of course, that will give us a DateTime object... and you can't just print a DateTime. But you can pipe it to the date() filter. Or in the last tutorial, we installed a library that allows us to say |ago.

97 lines | templates/question/show.html.twig
// ... lines 1 - 4
{% block body %}
<div class="container">
// ... lines 7 - 54
<ul class="list-unstyled">
{% for answer in question.answers %}
<li class="mb-4">
<div class="row">
// ... lines 59 - 65
<div class="col-2 text-end">
<small>{{ answer.createdAt|ago }}</small>
// ... lines 68 - 89
</div>
</div>
</li>
{% endfor %}
</ul>
</div>
{% endblock %}

When we refresh now... oh! We get an error:

The Question object cannot be found by the @ParamConverter annotation.

That's a fancy way of saying that no Question for the slug in the URL could be found in the database. And that's because I reloaded my fixtures. Go to the homepage, refresh... and click into a fresh question. Actually, let me try a different one... I want something with several answers. Perfect. And each answer does display how long ago it was added.

Ordering $question->getAnswers() with ORM\OrderBy

But this highlights a small problem... or question: what order are these answers being returned from the database? Right now... there's no specific order. You can see that in the query for the answers: it just queries for all the answers where question_id = ? this question... but there's no ORDER BY.

At first, it seems like this is one of the downsides of using the convenience methods for a relationship like $question->getAnswers(): you don't have a lot of control over the results. But... that's not entirely true.

The easiest thing that you can control is how the answers are ordered. Go into the Question class and scroll up to the $answers property. To control the order add @ORM\OrderBy() and pass this an array with {"createdAt" = "DESC"}.

179 lines | src/Entity/Question.php
// ... lines 1 - 14
class Question
{
// ... lines 17 - 51
/**
* @ORM\OneToMany(targetEntity=Answer::class, mappedBy="question")
* @ORM\OrderBy({"createdAt" = "DESC"})
*/
private $answers;
// ... lines 57 - 177
}

That's it! Go back, refresh and... perfect! These are now ordered with the newest first!

Optimizing The Query to Count a Relation: EXTRA_LAZY

Let's learn another trick. On the homepage, we show the number of answers for each question. Well... kind of: they all say 6 because that number is still hardcoded. Let's fix that.

Open the template for this: templates/question/homepage.html.twig... and I'll search for "6". Here it is. Replace this with {{ question.answers|length }}

50 lines | templates/question/homepage.html.twig
// ... lines 1 - 9
<div class="container">
// ... lines 11 - 15
<div class="row">
{% for question in questions %}
<div class="col-12 mb-3">
<div style="box-shadow: 2px 3px 9px 4px rgba(0,0,0,0.04);">
// ... lines 20 - 37
<a class="answer-link" href="{{ path('app_question_show', { slug: question.slug }) }}" style="color: #fff;">
<p class="q-display-response text-center p-3">
<i class="fa fa-magic magic-wand"></i> {{ question.answers|length}} answers
</p>
</a>
</div>
</div>
{% endfor %}
</div>
</div>
// ... lines 48 - 50

So we get the collection of answers and then count them. Simple enough! And if we try it... this works: two answers, six answers, eight answers.

But check out the web debug toolbar. Woh! We suddenly have a lot of queries. Click to open Doctrine's profiler. The first query is still for all of the question objects. But then, one-by-one it selects FROM answer WHERE question_id = ? a specific question. It does this for the first question, then it selects the answers for the next question... and the next and the next.

This is called the N+1 problem: We have 1 query that gives us all of the questions. Then, for each of the the N questions, when we ask for its answers, it makes another query. The total query count is the number of questions - N - plus 1 for the original.

We're going to talk more about the N+1 problem later and how to fix it. But there's kind of a bigger problem right now: we're querying for all of the answer data.... simply to count them! That's total overkill!

As soon as we access this answers property, Doctrine queries for all the data so that it can return all of the Answer objects. Normally, that's great - because we do want to use those Answer objects. But in this case... all we want to do is count them!

If you find yourself in this situation, there is a solution. In the Question class, at the end of the OneToMany(), pass a new option called fetch="" set to EXTRA_LAZY.

179 lines | src/Entity/Question.php
// ... lines 1 - 14
class Question
{
// ... lines 17 - 51
/**
* @ORM\OneToMany(targetEntity=Answer::class, mappedBy="question", fetch="EXTRA_LAZY")
* @ORM\OrderBy({"createdAt" = "DESC"})
*/
private $answers;
// ... lines 57 - 177
}

Watch what happens. Right now we have 21 queries. When we refresh, we still have 21 queries. But open up the profiler. The first query is still the same. But every query after just selects COUNT() FROM answer! Instead of querying for all of the answer data, it only counts them!

This is what fetch="EXTRA_LAZY" gets you. If Doctrine determines that you're accessing a relation... but you're only counting that relation - not actually trying to use its data - then it will create a "count" query instead of grabbing all the data.

That's awesome! So awesome that you might be wondering: why isn't this the default behavior? If I'm counting the relation, why would we ever want Doctrine to query for all of the data?

Well... EXTRA_LAZY isn't always a good thing. Go to a question show page. Having the EXTRA_LAZY actually causes an extra query here. Before that change, this page required 2 queries. Now it has 3. Check them out. First, it selects the question data. Then it counts the answers. And then it re-does that query to grab all the data for the answers. That second COUNT query is new... and, in theory, shouldn't be needed.

The problem is the order of the code in the template. You can see this in show.html.twig: before we loop over the answers and use their data, we first count them. So at this moment Doctrine says:

Hey! You want to count the answers! I'll make a quick COUNT query for that.

Then, a millisecond later, we loop over all the answers... and so we need their data anyways. This causes Doctrine to make the full query.

If we reversed the order of this code - where we loop and use the data first - Doctrine would avoid the extra COUNT query because it would already know how many answers it has because it just queried for their data.

All of this is probably not too important and I'm going to leave it. In general, don't overly worry about optimizing. In the real world, I use Blackfire on production to find what my real performance issues are.

Next: in addition to changing the order of the answers when we call $question->getAnswers(), we can also filter this collection to, for example, only return approved answers. Let's get that set up next.