<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Category extends Model
{
    protected $table = 'category';

    protected $fillable = [
        'title',
    ];

    public function content()
    {
        return $this->hasOne('App\Category', 'category_id');
    }
}
?>
--------------------------------------------------------------
<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Content extends Model
{
    protected $table = 'content';

    protected $fillable = [
        'user_id', 'title', 'introText', 'text', 'photo', 'category_id', 'toDate', 'status', 'frontPage', 'created_at', 'updated_at'
    ];

    public function category()
    {
        return $this->belongsTo('App\Content', 'id');
    }

    public function similarContent() {
        return $this->hasMany('App\SimilarContent');
    }

    protected static function boot() {
        parent::boot();

        static::deleting(function($content) { // before delete() method call this
             $content->similarContent()->delete();
             // do the rest of the cleanup...
        });
    }
}