Ruby on Rails. link_to doesn't raise an exception

Why does Rails create the path to the current page in the href atribute of the anchor element instead of raising an exception if I pass to the link_to meth开发者_高级运维od an instance variable which isn't associated with any resource (and equals nil)?

Here's an example:

Routes

# app/config/routes.rb
Example::Application.routes.draw do
  resource :example
end

HAML

-# app/views/examples/show.html.haml
%div
  = link_to 'Non-existent resource', @ne_resource

HTML

<!-- http://localhost/example -->
<div>
  <a href="/example">Non-existent resource</a>

Thanks.

Debian GNU/Linux 6.0.1;

Ruby 1.9.2;

Ruby on Rails 3.0.6.

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If you take a look at the link_to method it links to the url using the url_for method.

  def link_to(*args, &block)
    if block_given?
      options      = args.first || {}
      html_options = args.second
      link_to(capture(&block), options, html_options)
    else
      name         = args[0]
      options      = args[1] || {}
      html_options = args[2]

      html_options = convert_options_to_data_attributes(options, html_options)
      url = url_for(options) #THIS GETS CALLED

      href = html_options['href']
      tag_options = tag_options(html_options)

      href_attr = "href=\"#{html_escape(url)}\"" unless href
      "<a #{href_attr}#{tag_options}>#{html_escape(name || url)}</a>".html_safe
    end
  end

URL for

  def url_for(options = {})
    options ||= {}
    url = case options
    when String
      options
    when Hash  #THIS CASE IS TRUE
      options = options.symbolize_keys.reverse_merge!(:only_path => options[:host].nil?)
      super
    when :back
      controller.request.env["HTTP_REFERER"] || 'javascript:history.back()'
    else
      polymorphic_path(options)
    end

    url
  end

From the above, you can see that url_for is valid without options or with a nilClass, it is not designed to raise an exception. If you want errors when using link_to, then make sure to use the dynamic "path" helper methods, in the above case, new_example_path.

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I asked DHH (the creator of Rails) a closely related question about link_to's behavior when passed nil, nil arguments. I was hoping it would just not render an tag at all, instead of making me check for nils before calling it. He graciously replied:

https://twitter.com/dhh/status/198487578352156672

The essence of this answer applies to your question. It needs to do something when it is handed a nil. Gazler points out what's technically happening, but DHH's response shows a bit of the higher level "why?" The url_for method is fine with taking a nil argument, with the current page a sensible default.

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