CategorizablePivot.php

<?php

namespace Acme\Demo\Models;

use Model;
use October\Rain\Database\Pivot;
use Backend\Facades\BackendAuth;

class CategorizablePivot extends Pivot
{
    public function beforeCreate()
    {
        $this->created_by = BackendAuth::getUser()->id;
        $this->updated_by = BackendAuth::getUser()->id;
    }
    
    public function beforeUpdate()
    {
        $this->updated_by = BackendAuth::getUser()->id;
    }
}

Category.php

<?php

namespace Acme\Demo\Models;

use Model;

class Category extends Model
{
    /**
     * @var string The database table used by the model.
     */
    public $table = 'acme_demo_categories';
  
    /**
     * @var array Guarded fields
     */
    protected $guarded = ['*'];

    /**
     * @var array Fillable fields
     */
    protected $fillable = [];
  
    public $morphedByMany = [
        'posts'  => ['Acme\Demo\Models\Post', 'name' => 'categorizable'],
    ];
}

create_categorizables_table.php

<?php namespace Acme\Demo\Updates;

use Schema;
use October\Rain\Database\Schema\Blueprint;
use October\Rain\Database\Updates\Migration;

class CreateCategorizablesTable extends Migration
{
    public function up()
    {
        Schema::create('categorizables', function(Blueprint $table) {
            $table->engine = 'InnoDB';
            $table->increments('id');
            $table->integer('category_id')->unsigned();
            $table->integer('categorizable_id')->unsigned();
            $table->string('categorizable_type');
            $table->integer('created_by')->unsigned()->nullable();
            $table->integer('updated_by')->unsigned()->nullable();
            $table->timestamp('created_at')->nullable();
            $table->timestamp('updated_at')->nullable();
            $table->foreign('created_by')->references('id')->on('backend_users')->onDelete('set null');
            $table->foreign('updated_by')->references('id')->on('backend_users')->onDelete('set null');
            $table->foreign('category_id')->references('id')->on('acme_demo_categories')->onDelete('cascade');
        });
    }

    public function down()
    {
        Schema::dropIfExists('categorizables');
    }
}

Post.php

<?php

namespace Acme\Demo\Models;

use Model;

class Post extends Model
{
    /**
     * @var string The database table used by the model.
     */
    public $table = 'acme_demo_posts';
  
    /**
     * @var array Guarded fields
     */
    protected $guarded = ['*'];

    /**
     * @var array Fillable fields
     */
    protected $fillable = [];
  
    public $morphToMany = [
        'categories' => [
            'Acme\Demo\Models\Category',
            'name' => 'categorizable',
            'pivot' => ['id', 'created_by', 'updated_by'],
            'timestamps' => true,
            'pivotModel' => 'Acme\Demo\Models\CategorizablePivot',
        ],
    ];
}