public function getGrade()
  {
    return $this->hasOne(Grade::class, ['id' => 'value'])
      ->viaTable('employee_diffs', ['employee_id' => 'id'], function ($query)
      {
        return $query->andWhere(['employee_diffs.diff_type' => Diff::DIFF_TYPE_SALARY_GRADE_CATEGORY]);
      });
  }

  public function getDepartment()
  {
    return $this->hasOne(Department::class, ['id' => 'department_id'])->via('grade');
  }