Question

I have this RoR snippet in a view:

<%= link_to_unless(@posts_pages[:previous].nil?, 
  "Previous", 
  blog_with_page_path(:page_num => @posts_pages[:previous])) %>

Here blog_with_page is a named route. The snippet works if @posts_pages[:previous].nil? is false (as expected) and the link is generated correctly. However, when @posts_pages[:previous].nil? is true, instead of simply getting the "Previous" string back, I get an error telling me that the route couldn’t be generated using :page_num=>nil. Is this the expected behavior? If the condition is met, the route code shouldn’t be evaluated, should it?
Here’s the complete error:
blog_with_page_url failed to generate from {:page_num=>nil, :action=>"show", :controller=>"pages"}, expected: {:action=>"show", :controller=>"pages"}, diff: {:page_num=>nil}

I’ve been looking at the link_to_unless code and I don’t understand why I get the error since it should be returning simply the name:

# File actionpack/lib/action_view/helpers/url_helper.rb, line 394
  def link_to_unless(condition, name, options = {}, html_options = {}, &block)
    if condition
      if block_given?
        block.arity <= 1 ? capture(name, &block) : capture(name, options, html_options, &block)
      else
        name
      end
    else
      link_to(name, options, html_options)
    end
  end

I’m using Rails 2.3.11 and Ruby 1.8.7

Cheers!

Answer 1

Because Ruby is not a lazy language, blog_with_page_path(:page_num => @posts_pages[:previous]) gets evaluated as soon as you call it, regardless of whether the value ever gets used by link_to_unless.