How do I execute an if statement inside an image_tag helper in Rails 3?

Basically what I want to do is check to see if a method returns true on an object, if it does, then apply a specific class to the image.

If not, then do nothing.

So I tried this:

<%= image_tag(upload.image.url, if upload.upvoted? :class => 'upvoted') %>

But this is the error I get:

/app/views/stages/compare.html.erb:29: syntax error, unexpected ')', expecting keyword_then or ';' or '\n'
...d.upvoted? :class => 'upvoted') );@output_buffer.safe_concat...
...          开发者_JAVA百科                     ^
/app/views/stages/compare.html.erb:31: syntax error, unexpected keyword_end, expecting ')'
');              end                        
                    ^
/app/views/stages/compare.html.erb:63: syntax error, unexpected keyword_ensure, expecting ')'
/app/views/stages/compare.html.erb:65: syntax error, unexpected keyword_end, expecting ')'

---

<%= image_tag(upload.image.url, :class => upload.upvoted? ? 'upvoted' : nil) %>

---

As I said in my comment, I would move the logic for determining the class to a helper method:

module StageHelper
  def upload_class(upload)
    if upload.upvoted?
      'upvoted'
    elsif upload.downvoted?
      'downvoted'
    end
  end
end

Then your image tag help would be:

<%= image_tag(upload.image.url, :class => upload_class(upload) %>