mergeFillable(['parent_id']); } public function parent(): BelongsTo { return $this->belongsTo(static::class, 'parent_id'); } public function children(): HasMany { return $this->hasMany(static::class, 'parent_id'); } public function hasParent(): bool { return ! is_null($this->parent_id); } public function isParentOf(HasParent $child): bool { return $this->id === $child->parent_id; } }