fields.yaml

# ===================================
#  Form Field Definitions
# ===================================

fields:
    title:
        label: Title
        span: left
        type: text
    sluggable:
        label: Slug
        type: sluggable
        relationName: slugs
        preset:
            field: title
            type: slug
        span: right
        required: true

Post.php

<?php namespace Acme\Demo\Models;

use Model;
use BusyRoomsCMS\Backend\Traits\BaseTrait;

/**
 * Post Model
 */
class Post extends Model
{
    use BaseTrait;

    public $implement = ['@RainLab.Translate.Behaviors.TranslatableModel'];

    public $translatable = [
        'title',
    ];
  
      /**
     * @var array Guarded fields
     */
    protected $guarded = ['*'];

    /**
     * @var array Fillable fields
     */
    protected $fillable = [];
  
    public $morphMany = [
        'slugs' => ['BusyRoomsCMS\Backend\Models\Slug', 'name' => 'sluggable', 'delete' => true],
    ];
}