Add tests.

tests/Unit/KeyBy/Alias.php

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+<?php
+
+namespace Tests\Unit\KeyBy;
+
+use Zerotoprod\DataModel\DataModel;
+
+class Alias
+{
+    use DataModel;
+
+    public string $id;
+    public string $name;
+}

tests/Unit/KeyBy/KeyByTest.php

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+<?php
+
+namespace Tests\Unit\KeyBy;
+
+use PHPUnit\Framework\Attributes\Test;
+use Tests\TestCase;
+
+class KeyByTest extends TestCase
+{
+    #[Test] public function from(): void
+    {
+        $User = User::from([
+            'Aliases' => [
+                [
+                    'id' => 'jd1',
+                    'name' => 'John Doe',
+                ],
+                [
+                    'id' => 'js1',
+                    'name' => 'John Smith'
+                ],
+            ]
+        ]);
+
+        self::assertEquals('John Doe', $User->Aliases['jd1']->name);
+        self::assertEquals('John Smith', $User->Aliases['js1']->name);
+    }
+
+    #[Test] public function nested(): void
+    {
+        $User = User::from([
+            'AliasesNested' => [
+                [
+                    [
+                        'id' => 'jd1',
+                        'name' => 'John Doe',
+                    ],
+                    [
+                        'id' => 'js1',
+                        'name' => 'John Smith'
+                    ],
+                ],
+                [
+                    [
+                        'id' => 'jd1',
+                        'name' => 'John Doe',
+                    ],
+                    [
+                        'id' => 'js1',
+                        'name' => 'John Smith'
+                    ],
+                ]
+            ]
+        ]);
+
+        self::assertEquals('John Doe', $User->AliasesNested[0]['jd1']->name);
+        self::assertEquals('John Smith', $User->AliasesNested[0]['js1']->name);
+
+        self::assertEquals('John Doe', $User->AliasesNested[1]['jd1']->name);
+        self::assertEquals('John Smith', $User->AliasesNested[1]['js1']->name);
+    }
+}

tests/Unit/KeyBy/User.php

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+<?php
+
+namespace Tests\Unit\KeyBy;
+
+use Zerotoprod\DataModel\DataModel;
+use Zerotoprod\DataModel\Describe;
+use Zerotoprod\DataModelHelper\DataModelHelper;
+
+class User
+{
+    use DataModel;
+    use DataModelHelper;
+
+    /** @var Alias[] $Aliases */
+    #[Describe([
+        'cast' => [self::class, 'mapOf'],
+        'type' => Alias::class,
+        'key_by' => 'id'
+    ])]
+    public array $Aliases;
+
+    /** @var Alias[][] $AliasesNested */
+    #[Describe([
+        'cast' => [self::class, 'mapOf'],
+        'type' => Alias::class,
+        'level' => 2,
+        'key_by' => 'id'
+    ])]
+    public array $AliasesNested;
+}