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https://laravelista.com/lessons/mysql-on-delete-cascade-or-set-null

<?php

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

class CreatePostsTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('posts', function (Blueprint $table) {
            $table->increments('id');
            $table->integer('user_id')->unsigned()->index()->nullable();
            $table->integer('category_id')->unsigned()->index() ;
            $table->integer('photo_id')->unsigned()->index() ;
            $table->string('title') ;
            $table->text('body');
            $table->timestamps();

/*
Delete User Content when deleting a user
reference the user ID on  Users, cascade down and delete everything
If the 'User' of the post is deleted, delete all the posts that he has written".
------------------------------------------------------
id | user_id | category_id | photo_id | title | body |
--------|---------------------------------------------
        |
  ______| 
 |      
-V-------------------------------------------------
id | role_id | is_active | name | email | password
---------------------------------------------------
*/
           $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade') ;





        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('posts');
    }
}


=================== EXTRA =========================
after running the migration you will see the relationship at the end of the file
when you export the .sql file:

-- Constraints for table `tasks`
--
ALTER TABLE `tasks`
  ADD CONSTRAINT `tasks_project_id_foreign` FOREIGN KEY (`project_id`) REFERENCES `projects` (`id`) ON DELETE CASCADE;
COMMIT;