Laravel属于没有返回

I have a user model, which can have many reports, and a report model obviously belonging to a user, whenever one is created.

However when I use return $this->belongsTo('App\User') on the report model No user is returned even when I have the correct user_id on the report, and correct id on the user table.

User

protected $fillable = [
    'name', 'email', 'password',
];

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

Report

protected $fillable = [
    'user_id', 'title', 'detail',
];

public function user()
{
    return $this->belongsTo('App\User', 'user_id');
}

I've solved it simply by using $report->user, instead of calling it like a function via $report->user()