[9.x] Improves Pivot tests (#44626)

* [9.x] Improves accessing `$model->pivot->getKey()` doesn't throw an error on "strict mode" when pivot doesn't have an ID.

```
The attribute [id] either does not exist or was not retrieved for model [Illuminate\Database\Eloquent\Relations\Pivot]. {"userId":4,"exception":"[object] (Illuminate\\Database\\Eloquent\\MissingAttributeException(code: 0): The attribute [id] either does not exist or was not retrieved for model [Illuminate\\Database\\Eloquent\\Relations\\Pivot]. at /vendor/laravel/framework/src/Illuminate/Database/Eloquent/Concerns/HasAttributes.php:470)
````

Signed-off-by: Mior Muhammad Zaki <[email protected]>

* wip

Signed-off-by: Mior Muhammad Zaki <[email protected]>

Signed-off-by: Mior Muhammad Zaki <[email protected]>

Author: crynobone

tests/Integration/Database/EloquentPivotTest.php

@@ -0,0 +1,108 @@
+<?php
+
+namespace Illuminate\Tests\Integration\Database;
+
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\Pivot;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+class EloquentPivotTest extends DatabaseTestCase
+{
+    protected function defineDatabaseMigrationsAfterDatabaseRefreshed()
+    {
+        Schema::create('users', function (Blueprint $table) {
+            $table->increments('id');
+            $table->string('email');
+            $table->timestamps();
+        });
+
+        Schema::create('projects', function (Blueprint $table) {
+            $table->increments('id');
+            $table->string('name');
+            $table->timestamps();
+        });
+
+        Schema::create('collaborators', function (Blueprint $table) {
+            $table->integer('user_id');
+            $table->integer('project_id');
+            $table->text('permissions')->nullable();
+        });
+
+        Schema::create('contributors', function (Blueprint $table) {
+            $table->id();
+            $table->integer('user_id');
+            $table->integer('project_id');
+            $table->text('permissions')->nullable();
+        });
+    }
+
+    public function testPivotConvenientHelperReturnExpectedResult()
+    {
+        $user = PivotTestUser::forceCreate(['email' => '[email protected]']);
+        $user2 = PivotTestUser::forceCreate(['email' => '[email protected]']);
+        $project = PivotTestProject::forceCreate(['name' => 'Test Project']);
+
+        $project->contributors()->attach($user);
+        $project->collaborators()->attach($user2);
+
+        tap($project->contributors->first()->pivot, function ($pivot) use ($project, $user) {
+            $this->assertEquals(1, $pivot->getKey());
+            $this->assertEquals(1, $pivot->getQueueableId());
+            $this->assertSame('user_id', $pivot->getRelatedKey());
+            $this->assertSame('project_id', $pivot->getForeignKey());
+        });
+
+        tap($project->collaborators->first()->pivot, function ($pivot) use ($project, $user2) {
+            $this->assertNull($pivot->getKey());
+            $this->assertSame('project_id:1:user_id:2', $pivot->getQueueableId());
+            $this->assertSame('user_id', $pivot->getRelatedKey());
+            $this->assertSame('project_id', $pivot->getForeignKey());
+        });
+    }
+}
+
+class PivotTestUser extends Model
+{
+    public $table = 'users';
+}
+
+class PivotTestProject extends Model
+{
+    public $table = 'projects';
+
+    public function collaborators()
+    {
+        return $this->belongsToMany(
+            PivotTestUser::class, 'collaborators', 'project_id', 'user_id'
+        )->withPivot('permissions')
+        ->using(PivotTestCollaborator::class);
+    }
+
+    public function contributors()
+    {
+        return $this->belongsToMany(PivotTestUser::class, 'contributors', 'project_id', 'user_id')
+            ->withPivot('id', 'permissions')
+            ->using(PivotTestContributor::class);
+    }
+}
+
+class PivotTestCollaborator extends Pivot
+{
+    public $table = 'collaborators';
+
+    protected $casts = [
+        'permissions' => 'json',
+    ];
+}
+
+class PivotTestContributor extends Pivot
+{
+    public $table = 'contributors';
+
+    public $incrementing = true;
+
+    protected $casts = [
+        'permissions' => 'json',
+    ];
+}