ruby-on-rails - rails < 4.0 "try"方法抛出 NoMethodError？

Question

为什么尝试抛出错误？这不是违背了整个目的吗？也许它只是在控制台中？

ruby-1.9.2-p180 :101 > User.first.try(:something)
NoMethodError: undefined method `something' for #<User:0x000001046ad128>
    from /Users/me/.rvm/gems/ruby-1.9.2-p180/gems/activemodel-3.0.10/lib/active_model/attribute_methods.rb:392:in `method_missing'
    from /Users/me/.rvm/gems/ruby-1.9.2-p180/gems/activerecord-3.0.10/lib/active_record/attribute_methods.rb:46:in `method_missing'
    from (irb):101
    from /Users/me/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.10/lib/rails/commands/console.rb:44:in `start'
    from /Users/me/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.10/lib/rails/commands/console.rb:8:in `start'
    from /Users/me/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.10/lib/rails/commands.rb:23:in `<top (required)>'
    from script/rails:6:in `require'
    from script/rails:6:in `<main>'

编辑:

谢谢大家，现在我明白了。

有没有办法在不使用 respond_to? 的情况下做我想做的事，使得 User.try(:something)返回 nil而不是抛出错误？

Answer 1

rails 3

你怎么理解try作品，来自 fine manual :

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.

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.

以及 try that is patched into NilClass 的版本:

try(*args)
Calling try on nil always returns nil.

所以try不会忽略您在对象上调用不存在的方法的尝试，它会忽略您在 nil 上调用方法的尝试并返回 nil而不是引发异常。 try方法只是一种避免必须检查 nil 的简单方法。在方法调用链的每一步。

rails 4
try的行为在 Rails 4 中发生了变化，所以现在它:

Invokes the public method whose name goes as first argument just like public_send does, except that if the receiver does not respond to it the call returns nil rather than raising an exception.

所以现在try同时处理两项检查。如果你想要 Rails 3 的行为，有 try! :

Same as try, but will raise a NoMethodError exception if the receiving [sic] is not nil and does not implemented [sic] the tried method.