php – Laravel在Eloquent mutators中保存了多对多的关系

<?php

class Profile extends Eloquent {

    protected $fillable = [ 'name','photo','tags' ];
    protected $appends = [ 'tags' ];

    public function getTagsAttribute()
    {
        $tag_ids = [];
        $tags = $this->tags()->get([ 'tag_id' ]);

        foreach ($tags as $tag) {
            $tag_ids[] = $tag->tag_id;
        }

        return $tag_ids;
    }

    public function setTagsAttribute($tag_ids)
    {
        foreach ($tag_ids as $tag_id) {
            $this->tags()->attach($tag_id);
        }
    }

    public function tags()
    {
        return $this->belongsToMany('Tag');
    }

}

<?php

class Tag extends Eloquent {

    protected $fillable = [ 'title' ];
    protected $appends = [ 'profiles' ];

    public function getProfilesAttribute()
    {
        $profile_ids = [];
        $profiles = $this->profiles()->get([ 'profile_id' ]);

        foreach ($profiles as $profile) {
            $profile_ids[] = $profile->profile_id;
        }

        return $profile_ids;
    }

    public function profiles()
    {
        return $this->belongsToMany('Profile');
    }

}

但是setTagsAttribute函数没有按预期工作.我收到以下错误：SQLSTATE [23000]：完整性约束违规：1048列’profile_id’不能为空(SQL：insert intoprofile_tag(profile_id,tag_id)值(？,？))(绑定：数组(0 => ; NULL,1 => 1,))