rails try方法抛出NoMethodError？

Question

为什么尝试抛出错误？不是打败了整个目的吗？也许只是在控制台？

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:

谢谢你们,现在我明白了.

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

Answer 1

Rails 3

你try从错误的手册中误解了它是如何工作的:

try(*a,&b)
调用symbol方法标识的方法,传递任何参数和/或指定的块,就像常规Ruby Object#send一样.

但是,与该方法不同的是,如果接收对象是对象或,则NoMethodError不会引发异常并将nil返回异常.nilNilClass

并将其版本try修补为NilClass:

尝试(*参数)
调用try上nil总是返回nil.

因此try,不会忽略您在对象上调用不存在的方法的尝试,它会忽略您尝试调用方法nil并返回nil而不是引发异常.该try方法只是一种简单的方法,可以避免nil在方法调用链的每一步检查.

Rails 4

tryRails 4中的行为发生了变化,现在它:

调用名称与第一个参数一样的公共方法public_send,只是如果接收者没有响应它,则调用返回nil而不是引发异常.

所以现在立即try处理这两项检查.如果你想要Rails 3的行为,有try!:

相同try,但将引发一个NoMethodError例外,如果在接收[原文如此]是不nil,不执行[原文如此]的尝试方法.

Answer 2

这就是尝试

调用symbol方法标识的方法,传递任何参数和/或指定的块,就像常规的Ruby Object #send一样.但是,与该方法不同,如果接收对象是nil对象或NilClass,则不会引发NoMethodError异常并返回nil.

所以,假设您@user在控制器中进行了设置,但是您没有实例化它@user.try(:foo) => nil而不是

@user.foo
NoMethodError: undefined method `foo' for nil:NilClass

这里重要的一点是try是一个实例方法.如果您尝试的对象不是nil,它也不会返回nil.

Answer 3

我知道这是旧的,但它可能会帮助其他人,因为这是我搜索这个问题时出现的第一件事.我"借用"了try的代码并实现了我自己的try_method方法,该方法就像try一样,除了它在调用send之前首先检查方法是否存在.我在Object中实现了它并将它放在初始化器中,现在我可以在任何对象上调用它.

class Object
  # Invokes the method identified by _method_, passing it any
  # arguments specified, just like the regular Ruby <tt>Object#send</tt> does.
  #
  # *Unlike* that method however, a +NoMethodError+ exception will *not* be raised
  # if the method does not exist.
  #
  # This differs from the regular Ruby <tt>Object#try</tt> method which only
  # suppresses the +NoMethodError+ exception if the object is Nil
  #
  # If try_method is called without a method to call, it will yield any given block with the object.
  #
  # Please also note that +try_method+ is defined on +Object+, therefore it won't work with
  # subclasses of +BasicObject+. For example, using try_method with +SimpleDelegator+ will
  # delegate +try_method+ to target instead of calling it on delegator itself.
  #
  # ==== Examples
  #
  # Without +try_method+
  #   @person && @person.respond_to?(:name) && @person.name
  # or
  #   (@person && @person.respond_to?(:name)) ? @person.name : nil
  #
  # With +try_method+
  #   @person.try_method(:name)
  #
  # +try_method+ also accepts arguments and/or a block, for the method it is trying
  #   Person.try_method(:find, 1)
  #   @people.try_method(:collect) {|p| p.name}
  #
  # Without a method argument try_method will yield to the block unless the receiver is nil.
  #   @person.try_method { |p| "#{p.first_name} #{p.last_name}" }
  #--
  # +try_method+ behaves like +Object#send+, unless called on +NilClass+ or a class that does not implement _method_.
  def try_method(method=nil, *args, &block)
    if method == nil && block_given?
      yield self
    elsif respond_to?(method)
      __send__(method, *args, &block)
    else
      nil
    end
  end
end

class NilClass
  # Calling +try_method+ on +nil+ always returns +nil+.
  # It becomes specially helpful when navigating through associations that may return +nil+.
  #
  # === Examples
  #
  #   nil.try_method(:name) # => nil
  #
  # Without +try_method+
  #   @person && @person.respond_to(:children) && !@person.children.blank? && @person.children.respond_to(:first) && @person.children.first.respond_to(:name) && @person.children.first.name
  #
  # With +try_method+
  #   @person.try_method(:children).try_method(:first).try_method(:name)
  def try_method(*args)
    nil
  end
end