Laravel 关系在急切加载时显示为空

Question

A Laravel relation is displaying null on eager load. However, it works when the relation is accessed normally.

class Student extends Model
{
    use SoftDeletes;

    public $incrementing = false;
    protected $primaryKey = 'id'; // or null
    protected $guarded = [];

    public function document()
    {
        return $this->hasOne('App\Model\Document');
    }

    public function contact()
    {
        return $this->hasOne('App\Model\Contact');
    }
}

The contract relation returns null when I use the following.

Student::with('contact')->get()

However it works when I do the following. What could be the cause of this?

$student = Student::findOrFail($id);
$contact = $student->contact;

Answer 1

Student::with('contact')->get(); returns a Collection of Student instances, which you can loop and access the contact relationship:

$students = Student::with('contact')->get();
foreach($students AS $student){
  dd($student->contact); 
  // Can be `null` or an instance of `Contact`
}

When you call Student::findOrFail($id); , you're getting a single instance of Student , which you can directly access contact :

$student = Student::with('contact')->findOrFail($id);
dd($student->contact);
// Again, can be `null` or an instance of `Contact`

The with() clause is eager loading, and doesn't do anything until you try to access $student->contact , but due to the nature of hasOne() , it can be null .