属性更新为新值或为nil时，Rails触发模型回调

Question

I'm trying to get a before_update model callback working where quote_file_date is automatically timestamped based off whether quote_file is created, updated or removed. The purpose of this is so that I can track when the file is created and updated.

I'm unsure how to implement ActiveModel::Dirty to get this working properly, but the expected behaviour should be:

1. When quote_file is created, a quote_file_date timestamp is created.
2. When quote_file value changes, the quote_file_date timestamp is updated.
3. If quote_file is removed, the quote_file_date is set back to nil.

Currently, I have:

class Job < ActiveRecord::Base

  before_update :quote_file_updated?, on: [:create, :update], if: quote_file.identifier.changed?

  private

  def quote_file_updated?
    self.quote_file_date = Time.now
  end

end

And the error I get back from this is:

undefined method `quote_file' for #<Class:0x007fa3bbedfec0>

What is the most elegant way to achieve this using callbacks in the model?

Answer 1

答案是将before_update回调更改为：

before_update :quote_file_updated?, on: [:create, :update], if: ->(r) { r.quote_file_changed? }