接下來要講的是Eloquent的Has One Through/Has Many Through的關係，這兩個關係相對來說少用，但是在三個Model的狀況時，兩個不連結的Model可以透過中間Model而連結起來，提供便利的效果。

我們用官方文件（Laravel 8）提供的例子來試做Has One Through/Has Many Through的關係：

1. Has One Through：Mechanic擁有一個Car，Car擁有一個Owner
   • 資料表

          class CreateMechanicsTable extends Migration
  {
     /**
      * Run the migrations.
      *
      * @return void
      */
     public function up()
     {
         Schema::create('mechanics', function (Blueprint $table) {
             $table->id();
             $table->string('name');
             $table->timestamps();
         });
     }

           class CreateCarsTable extends Migration
   {
      /**
       * Run the migrations.
       *
       * @return void
       */
      public function up()
      {

          Schema::create('cars', function (Blueprint $table) {
              $table->id();
         $table->foreignId('mechanic_id')->constrained('mechanics')->onDelete('cascade');
              $table->string('model');
              $table->timestamps();
          });
      }

            class CreateOwnersTable extends Migration
    {
       /**
        * Run the migrations.
        *
        * @return void
        */
       public function up()
       {
           Schema::create('owners', function (Blueprint $table) {
               $table->id();
               $table->string('name');
               $table->foreignId('car_id')->constrained('cars')->onDelete('cascade');
               $table->timestamps();
           });
       }

• Model

         class Mechanic extends Model
   {
      use HasFactory;
      public function car()
      {
          return $this->hasOne('App\Models\Car');
      }
      public function carOwner()
      {
          return $this->hasOneThrough('App\Models\Owner', 'App\Models\Car');
      }
   }

         class Car extends Model
   {
      use HasFactory;
      public function owner()
      { 
          return $this->hasOne('App\Models\Owner');
      }

      public function mechanic()
      {
          return $this->belongsTo('App\Models\Mechanic');
      }
   }

         class Owner extends Model
   {
      use HasFactory;
      public function car()
      {
          return $this->belongsTo('App\Models\Car');
      }
   }

• 使用php artisan test 測試：結果通過

         class HaveThroughTest extends TestCase
   {
      use RefreshDatabase;
      /**
       * A basic feature test example.
       *
       * @test
       */
      public function mechanic_have_one_owner_through_car()
      {
          $car = Car::factory()->hasOwner(1)->create();
          $mechanic = $car->mechanic;
          $owner = $car->owner;
          $this->assertTrue($owner->is($mechanic->carOwner));
      }
   ...
   }

2. Has Many Through：Country擁有很多Visitor，Visitor擁有很多Post
   • 資料表

         class CreateCountriesTable extends Migration
   {
      /**
       * Run the migrations.
       *
       * @return void
       */
      public function up()
      {
          Schema::create('countries', function (Blueprint $table) {
              $table->id();
              $table->string('name');
              $table->timestamps();
          });
      }

       class CreateVisitorsTable extends Migration
  {
     /**
      * Run the migrations.
      *
      * @return void
      */
     public function up()
     {
         Schema::create('visitors', function (Blueprint $table) {
             $table->id();
             $table->foreignId('country_id')->constrained()->onDelete('cascade');
             $table->string('name');
             $table->timestamps();
         });
     }

        class CreatePostsTable extends Migration
  {
     /**
      * Run the migrations.
      *
      * @return void
      */
     public function up()
     {
         Schema::create('posts', function (Blueprint $table) {
             $table->id();
             $table->foreignId('visitor_id')->constrained()->onDelete('cascade');
             $table->string('title');
             $table->timestamps();
         });
     }

• Model

        class Country extends Model
  {
     use HasFactory;
     public function visitors()
     {
         return $this->hasMany('App\Models\Visitor');
     }

     public function posts()
     {
         return $this->hasManyThrough('App\Models\Post','App\Models\Visitor');
     }
  }

        class Visitor extends Model
  {
     use HasFactory;
     public function country()
     {
         return $this->belongsTo('App\Models\Country');
     }
     public function posts()
     {
         return $this->hasMany('App\Models\Post');
     }
  }

        class Post extends Model
  {
     use HasFactory;
     public function visitor()
     {
         return $this->belongsTo('App\Models\Visitor');
     }

  }

• 使用php artisan test 測試：結果通過了

        class HaveThroughTest extends TestCase
  {
  ...
  public function country_has_many_postst_hrough_visitors()
     {
         $country = Country::factory()->hasVisitors(4)->create();
         $visitors = $country->visitors;
         foreach($visitors as $visitor){
             Post::factory()->count(2)->create(['visitor_id'=>$visitor->id]);
         }
         $this->assertCount(8,$country->posts);
     }
  }