Ruby-on-rails Rails：如何只为一个更改的属性运行 before_update？
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Rails: How to run before_update only for one changed attribute?
提问by serial engine
In my model ShopI'm saving image url in logo_oriand use that to make thumbnails using before_update.
在我的模型中，Shop我将图像 url 保存在其中logo_ori并使用它来制作缩略图before_update。
# shop.rb
before_update :run_blitline_job
private
def run_blitline_job
# uses logo_ori to make thumbnails
end
However I found out that when I'm saving other attributes (eg: editing shop's profile in a form) it also runs before_update. How do I confine its execution when only logo_oriis saved?
但是我发现当我保存其他属性（例如：在表单中编辑商店的个人资料）时，它也会运行before_update. 仅logo_ori在保存时如何限制其执行？
I've tried this :
我试过这个：
before_update :run_blitline_job, :if => :logo_ori?
but it still runs before_updateif I already have logo_orisaved earlier.
但before_update如果我之前已经logo_ori保存过，它仍然会运行。
回答by John H
before_update :run_blitline_job, :if => :logo_ori_changed?
This will run the callback every time the logo_oriattribute changes. You can also use strings to implement multiple conditionals:
这将在每次logo_ori属性更改时运行回调。您还可以使用字符串来实现多个条件：
before_update :run_blitline_job, :if => proc { !logo_ori_was && logo_ori_changed? }
回答by Brad Werth
You are close, you want something like this:
你很接近，你想要这样的东西：
before_update { |shop| shop.run_blitline_job if shop.logo_ori_changed? }
sources:
来源：
http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html
http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html
回答by Muhamamd Awais
its simple you can use ActiveModel::Dirty(checkout the documentation), it is available in all models in rails 3
它很简单，您可以使用ActiveModel::Dirty（查看文档），它适用于 rails 3 中的所有模型
before_update { |shop| shop.run_blitline_job if shop.logo_ori_changed? }
