Rails 3 对象#尝试不工作? [英] Rails 3 Object#try not working?

问题描述

我应该从哪里开始寻找?这就是让我相信这一点的原因:

Where should I start looking? Here's what makes me believe that:

0 urzatron work/secret_project % rails c
Loading development environment (Rails 3.1.3)

irb(main):001:0> t = Tag.new(:name => "!Blark!")
=> #<Tag id: nil, name: "!Blark!", created_at: nil, updated_at: nil>

irb(main):002:0> t.try(:name)
=> "!Blark!"

irb(main):003:0> t.try(:aoeu)
NoMethodError: undefined method `aoeu' for #<Tag id: nil, name: "!Blark!", created_at: nil, updated_at: nil>
        from /usr/lib/ruby/gems/1.9.1/gems/activemodel-3.1.3/lib/active_model/attribute_methods.rb:385:in `method_missing'
        from /usr/lib/ruby/gems/1.9.1/gems/activerecord-3.1.3/lib/active_record/attribute_methods.rb:60:in `method_missing'
        from /usr/lib/ruby/gems/1.9.1/gems/activesupport-3.1.3/lib/active_support/core_ext/object/try.rb:32:in `try'
        from (irb):3
        from /usr/lib/ruby/gems/1.9.1/gems/railties-3.1.3/lib/rails/commands/console.rb:45:in `start'
        from /usr/lib/ruby/gems/1.9.1/gems/railties-3.1.3/lib/rails/commands/console.rb:8:in `start'
        from /usr/lib/ruby/gems/1.9.1/gems/railties-3.1.3/lib/rails/commands.rb:40:in `<top (required)>'
        from script/rails:6:in `require'
        from script/rails:6:in `<main>'

Tag 模型:

class Tag < ActiveRecord::Base
  has_many :taggings, :dependent => :destroy
end

推荐答案

您误解了 try 的作用.来自精美手册:

You're misunderstanding what try does. From the fine manual:

尝试(*a, &b)
调用由符号 method 标识的方法，将任何参数和/或指定的块传递给它，就像常规的 Ruby Object#send 一样.

try(*a, &b)
Invokes the method identified by the symbol method, passing it any arguments and/or the block specified, just like the regular Ruby Object#send does.

然而，与该方法不同的是，如果接收对象是 NoMethodError 异常，不会 被引发，并且 nil 将被返回code>nil 对象或 NilClass.

Unlike that method however, a NoMethodError exception will not be raised and nil will be returned instead, if the receiving object is a nil object or NilClass.

这样做:

t.try(:aoeu)

或多或少与此相同:

t.nil?? nil : t.aoeu

但您似乎希望它表现得像这样:

but you seem to be expecting it to behave like this:

t.respond_to?(:aoeu) ? t.aoeu : nil

你的 t 不是 nil 所以 t.try(:aoeu) 和 t.aoeu 是一样的代码>.您的 Tag 类没有 aoeu 方法，因此您会得到 NoMethodError.

Your t isn't nil so t.try(:aoeu) is the same as t.aoeu. Your Tag class doesn't have an aoeu method so you get a NoMethodError.

try 只是一种避免 nil 检查的便捷方法，当对象不存在时，它不是一种避免 NoMethodError 的方法不回应您尝试使用的方法.

try is just a convenient way of avoiding a nil check, it isn't a way to avoid a NoMethodError when the object doesn't respond to the method you're trying to use.

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