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Form Events & Dynamic ChoiceType choices

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Let's focus on the edit form first - it'll be a little bit easier to get working. Go to /admin/article and click to edit one of the existing articles. So, based on the location, we need to make this specificLocationName field have different options.

Determining the specificLocationName Choices

Open ArticleFormType and go to the bottom. I'm going to paste in a function I wrote called getLocationNameChoices(). You can copy this function from the code block on this page. But, it's fairly simple: We pass it the $location string, which will be one of solar_system, star or interstellar_space, and it returns the choices for the specificLocationName field. If we choose "solar system", it returns planets. If we choose "star", it returns some popular stars. And if we choose "Interstellar space", it returns null, because we actually don't want the drop-down to be displayed at all in that case.

108 lines | src/Form/ArticleFormType.php
// ... lines 1 - 75
private function getLocationNameChoices(string $location)
$planets = [
$stars = [
'Alpha Centauari A',
'Alpha Centauari B',
$locationNameChoices = [
'solar_system' => array_combine($planets, $planets),
'star' => array_combine($stars, $stars),
'interstellar_space' => null,
return $locationNameChoices[$location];
// ... lines 107 - 108

Oh, and I'm using array_combine() just because I want the display values and the values set back on my entity to be the same. This is equivalent to saying 'Mercury' => 'Mercury'... but saves me some duplication.

Dynamically Changing the Options

The first step to get this working is not so different from something we did earlier. To start, forget about trying to use fancy JavaScript to instantly reload the specificLocationName drop-down when we select a new location. Yes, we are going to do that - but later.

Hit "Update" the save the location to "The Solar System". The first goal is this: when the form loads, because the location field is already set, the specificLocationName should show me the planet list. In other words, we should be able to use the underlying Article data inside the form to figure out which choices to use.

I'll add some inline documentation just to tell my editor that this is an Article object or null. Then, $location = , if $article is an object, then $article->getLocation(), otherwise, null.

108 lines | src/Form/ArticleFormType.php
// ... lines 1 - 24
public function buildForm(FormBuilderInterface $builder, array $options)
/** @var Article|null $article */
$article = $options['data'] ?? null;
// ... line 29
$location = $article ? $article->getLocation() : null;
// ... lines 31 - 65
// ... lines 67 - 108

Down below, copy the entire specificLocationName field and remove it. Then only if ($location) is set, add that field. For choices, use $this->getLocationNameChoices() and pass that $location.

108 lines | src/Form/ArticleFormType.php
// ... lines 1 - 24
public function buildForm(FormBuilderInterface $builder, array $options)
// ... lines 27 - 52
if ($location) {
$builder->add('specificLocationName', ChoiceType::class, [
'placeholder' => 'Where exactly?',
'choices' => $this->getLocationNameChoices($location),
'required' => false,
// ... lines 60 - 65
// ... lines 67 - 108

Cool! Again, no, if we change the location field, it will not magically update the specificLocationName field... not yet, at least. With this code, we're saying: when we originally load the form, if there is already a $location set on our Article entity, let's add the specificLocationName field with the correct choices. If there is no location, let's not load that field at all, which means in _form.html.twig, we need to render this field conditionally: {% if articleForm.specificLocationName is defined %}, then call form_row().

16 lines | templates/article_admin/_form.html.twig
{{ form_start(articleForm) }}
// ... lines 2 - 6
{% if articleForm.specificLocationName is defined %}
{{ form_row(articleForm.specificLocationName) }}
{% endif %}
// ... lines 10 - 15
{{ form_end(articleForm) }}

Let's try this! Refresh the page. The Solar System is selected and so... sweet! There is our list of planets! And we can totally save this. Yep! It saved as Earth. Open a second tab and go to the new article form. No surprise: there is no specificLocationName field here because, of course, the location isn't set yet.

Our system now... sort of works. We can change the data... but we need to do it little-by-little. We can go to "Near a Star", hit "Update" and then change the specificLocationName field and save that. But I can't do it all at once: I need to fully reload the page... which kinda sucks!

Can you Hack the Options to Work?

Heck, we can't even be clever! Change location to "The Solar System". Then, inspect element on the next field and change the "Betelgeuse" option to "Earth". In theory, that should work, right? Earth is a valid option when location is set to solar_system, and so this should at least be a hacky way to work with the system.

Hit Update. Woh! It does not work! We get a validation error: This value is not valid. Why?

Think about it: when we submit, Symfony first builds the form based on the Article data that's stored in the database. Because location is set to star in the database, it builds the specificLocationName field with the star options. When it sees earth being submitted for that field, it looks invalid!

Our form needs to be even smarter: when we submit, the form needs to realize that the location field changed, and rebuild the specificLocationName choices before processing the data. Woh.

We can do that by leveraging form events.