foreign 外键级联 好像不需要

use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class AddForeignTagIdToArticlesTable extends Migration {

   /**
    * Run the migrations.
    *
    * @return void
    */
   public function up()
   {
      Schema::table('articles', function(Blueprint $table)
      {
//          $table->foreign('tag_id')
//          ->references('tag_id')
//          ->on('tags');
      });
   }

   /**
    * Reverse the migrations.
    *
    * @return void
    */
   public function down()
   {
//    Schema::table('articles', function(Blueprint $table)
//    {
//       $table->dropForeign('articles_tag_id_foreign');
//    });
   }

}