是否有办法在预加载中加载在中间模型中定义的关联，以避免N+1查询？

经过查阅文档和大量文章，我得到了以下4 个模型：

• Student
• Assessment
• StudentAssessment（M-N中间表）
• AssessmentResult（与StudentAssessment的一对一关系）

每个模型的关系：

• Student => assessments(): BelongsToMany
• Assessment => students(): BelongsToMany
• StudentAssessment => student(): BelongsTo, assessments(): BelongsTo, assessment_result(): HasOne
• AssessmentResult: student_assessment(): BelongsTo

StudentAssessment与 AssessmentResult都继承自Pivot而不是Model。students 关系在 Assessment 模型上也明确使用 withPivot 和 using:

  public function students(): BelongsToMany  
  {  
    return $this->belongsToMany(  
      Student::class,  
      'student_assessments',  
      'assessment_id',  
      'student_id'     
    )->using(StudentAssessment::class)  
      ->withPivot('id');  
  }

我仍然无法实现 $assessment->students()->with('pivot.assessment_result')。在预加载路径中，Laravel对中间表关系并不友好。

当然，我可以使用 foreach > load() 的模式逐个加载结果。只是我想避免那样做。

解决方案

你可以通过将pivot转换为自定义pivot模型来实现—

class StudentAssessment extends Pivot
{
    protected $table = 'student_assessment';

    public function assessment_result()
    {
        return $this->hasOne(AssessmentResult::class, 'student_assessment_id');
    }
}

在Assessment模型上定义关系 -

class Assessment extends Model
{
    public function students()
    {
        return $this->belongsToMany(Student::class)
            ->using(StudentAssessment::class)
            ->withPivot(['id']);
    }
}

现在你可以在pivot模型上定义关系，这些关系在通过父关系对pivot模型进行水合后就可用 -

$assessments = Assessment::with('students')->get();

用法 -

foreach ($assessments as $assessment) {
    foreach ($assessment->students as $student) {
        $result = $student->pivot->assessment_result;
    }
}