35.1.Setup: For Dependent Select Fields{Adding the First Select Field}

We're going to tackle one of the most annoying things in Symfony's form system, and, I hope, make it as painless as possible... because the end result is pretty cool!
Log in as admin2@thespacebar.com, password engage and then go to /admin/article. Click to create a new article. Here's the goal: on this form, I want to add two new drop-down select elements: a location drop-down - so you can choose where in the galaxy you are - and a second dropdown with more specific location options depending on what you chose for the location. For example, if you select "Near a star" for your location, the next drop-down would update to be a list of stars. Or, if you select "The Solar System", the next drop-down will be a list of planets.

Adding the First Select Field
This is called a "dependent form field", and, unfortunately, it's one of the trickier things to do with the form system - which is exactly why we're talking about it! Let's add the first new field. Find your terminal and run

php bin/console make:entity
Modify the Article entity and create a new field called location. Make it a string field with yes to nullable in the database: the location will be optional. Now run:

php bin/console make:migration
and open the Migrations/ directory to check out that new file.
No surprises here, so let's go back and run it:

php bin/console doctrine:migrations:migrate
Perfect!

src/Entity/Article.php

@@ -75,6 +75,11 @@ class Article{
 	 */
 	private $author;
 
+	/**
+	 * @ORM\Column(type="string", length=255, nullable=true)
+	 */
+	private $location;
+
 	public function __construct(){
 		$this->comments = new ArrayCollection();
 		$this->tags = new ArrayCollection();
@@ -239,4 +244,14 @@ public function validate(ExecutionContextInterface $context, $payload){
 				->addViolation();
 		}
 	}
+
+	public function getLocation(): ?string{
+		return $this->location;
+	}
+
+	public function setLocation(?string $location): self{
+		$this->location = $location;
+
+		return $this;
+	}
 }

src/Migrations/Version20200209135332.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DoctrineMigrations;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+/**
+ * Auto-generated Migration: Please modify to your needs!
+ */
+final class Version20200209135332 extends AbstractMigration {
+	public function getDescription() : string {
+		return '';
+	}
+
+	public function up(Schema $schema) : void {
+		// this up() migration is auto-generated, please modify it to your needs
+		$this->abortIf($this->connection->getDatabasePlatform()->getName() !== 'mysql', 'Migration can only be executed safely on \'mysql\'.');
+
+		$this->addSql('ALTER TABLE article ADD location VARCHAR(255) DEFAULT NULL');
+	}
+
+	public function down(Schema $schema) : void {
+		// this down() migration is auto-generated, please modify it to your needs
+		$this->abortIf($this->connection->getDatabasePlatform()->getName() !== 'mysql', 'Migration can only be executed safely on \'mysql\'.');
+
+		$this->addSql('ALTER TABLE article DROP location');
+	}
+}