ruby - each_value が NoMethodError を発生させる

Question

次のコードを含むモデルがあります。

appraisal_detail.each_value { |detail|
  # some code...
}

「undefined method 'each_value'」というメッセージで NoMethodError が発生しています。私は何か間違ったことをしていますか？どんな提案でも大歓迎です。

エラーが発生するメソッドの完全な内容は次のとおりです。

def self.update_appraisal_details(appraisal, appraisal_detail)
  status = true

  appraisal_detail_ids = appraisal.employee_appraisal_detail_ids

  appraisal_detail.each_value { |detail| 
    appraisal_detail = find_by_employee_appraisal_id_and_kra_list_id(appraisal.id, detail[:kra_list_id])

    if appraisal_detail.nil? # For newly added KRA
      new_detail = EmployeeAppraisalDetail.new(detail.merge({:employee_appraisal_id => appraisal.id}))
      status = (new_detail.save && status)
    elsif appraisal_detail.status == "Inactive" 
      status = (appraisal_detail.update_attributes(:status => "Active") && status)
      appraisal_detail_ids.delete(appraisal_detail.id)
    else 
      appraisal_detail_ids.delete(appraisal_detail.id)
    end
  }
end