如何在 Rails 中使用 instance_eval 覆盖 hash 常量

Question

如何使用插件中的instance_eval覆盖 class 中的常量？ 这是我要覆盖的常量：

class MessageTemplate < ActiveRecord::Base

     TEMPLATE_KEYS = {

    :guardian=>{
      :guardian_full_name=>"guardian_full_name",
      :guardian_first_name=>"guardian_first_name",
      :guardian_last_name=>"guardian_last_name",
      :ward_full_name=>"ward_full_name",
      :guardians_relation=>"guardians_relation",
      :ward_batch_name=>"ward_batch_name",
      :ward_admission_number=>"ward_admission_number",
      :guardian_email=>"guardian_email",
      :guardian_mobile_phone_no=>"guardian_mobile_phone_no"
    }

  }
end

我想将这些值从 Ruby on Rails 中的插件添加到上述常量。 这是我尝试过的，但失败了：

module IndepthEinvoice

         module MessageTemplate
    def self.included(base)
      base.instance_eval do
        self.TEMPLATE_KEYS = {
          :guardian=>{
              :payment_link=>"payment_link",
              :ward_balance_fee=>"ward_balance_fee"
          }
      }
      end
    end

Answer 1

修改重复或克隆而不是常量

Ruby 实际上不会阻止您重新分配常量； 它通常只会发出警告。 尽管如此，故意修改常量通常被认为是不好的形式。 相反，您应该使用Hash#merge或Hash#merge！ 将您的更改存储在本地 Hash object 中。 例如：

TEMPLATE_KEYS = {
  guardian: {
    guardian_full_name:       "guardian_full_name",
    guardian_first_name:      "guardian_first_name",
    guardian_last_name:       "guardian_last_name",
    ward_full_name:           "ward_full_name",
    guardians_relation:       "guardians_relation",
    ward_batch_name:          "ward_batch_name",
    ward_admission_number:    "ward_admission_number",
    guardian_email:           "guardian_email",
    guardian_mobile_phone_no: "guardian_mobile_phone_no",
  }
}

# make a copy of your TEMPLATE_KEYS constant
@my_template = TEMPLATE_KEYS.dup

# merge your modifications into an instance variable
@my_template[:guardian].merge!(
  {
    payment_link:     "payment_link",
    ward_balance_fee: "ward_balance_fee",
  }
)

@my_template.dig :guardian, :payment_link
#=> "payment_link"

当然还有其他方法可以做到这一点，有些方法比其他方法更优雅。 不过，这个原则是合理的，并且绝对比尝试直接修改常量更好，这可能会对您的应用程序产生意想不到的后果。