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https://stackoverflow.com/questions/26820788/add-on-delete-cascade-to-existing-column-in-laravel

1) first projects needs to be created :

<?php

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

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

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




2) then the tasks table

<?php /// 2017_08_19_154845_create_tasks_table

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

class CreateTasksTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('tasks', function (Blueprint $table) {
            $table->increments('id');
            $table->integer('project_id')->unsigned(); // THIS NEEDS TO BE UNSIGNED, its a foreign key
            $table->string('task_title');
            $table->text('task') ;
            $table->integer('priority')->default(0) ;
            $table->boolean('completed')->default(0) ;            
            $table->timestamps();
        });
/*
Delete Project Tasks Content when deleting a project
reference the project ID on  Projects, cascade down and delete everything
If the 'Project'  is deleted, delete all the tasks that belong to that project".
*/
        // THIS DOES THE MAGIC
        Schema::table('tasks', function (Blueprint $table) {
            $table->foreign('project_id')->references('id')->on('projects')->onDelete('cascade') ;
        });
    }

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



=================== 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;