Being something I do seldom this little gotcha caught me today.
So, you want to put a link into an email template or a rendered pdf or something similar that requires the full url of your route. Being an email or a pdf it is also likely that the contents will be printed to just rendering our the entire url in the template can be a good thing. However...
# the basic
# is not the same as
link_to something_url, something_url
The first one looks good. You get the full url including https:// and all. The gotcha is that Rails, being the opinionated and free-thinking framework it is, will strip out the url-part and just put the path part into the href. To make the visible link match the href you actually need to tell Rails explicitly that's what you want.