public function getEmployees()
  {
    return $this->hasMany(Employee::class, ['id' => 'employee_id'])->via('employeeDiffs');
  }

  /**
   * Used for chained relation with Employee
   *
   * @return ActiveQuery
   */
  public function getEmployeeDiffs()
  {
    return $this->hasMany(Diff::class, ['value' => 'id'])
      ->via('grades')
      ->andOnCondition(['employee_diffs.diff_type' => Diff::DIFF_TYPE_SALARY_GRADE_CATEGORY]);
  }

  public function getGrades(): ActiveQuery
  {
    return $this->hasMany(Grade::class, ['department_id' => 'id']);
  }