Add http(s) to URL if it's not there?

Use a before filter to add it if it is not there:

before_validation :smart_add_url_protocol

protected

def smart_add_url_protocol
  unless url[/\Ahttp:\/\//] || url[/\Ahttps:\/\//]
    self.url = "http://#{url}"
  end
end

Leave the validation you have in, that way if they make a typo they can correct the protocol.

Don't do this with a regex, use URI.parse to pull it apart and then see if there is a scheme on the URL:

u = URI.parse('/pancakes')
if(!u.scheme)
  # prepend http:// and try again
elsif(%w{http https}.include?(u.scheme))
  # you're okay
else
  # you've been give some other kind of
  # URL and might want to complain about it
end

Using the URI library for this also makes it easy to clean up any stray nonsense (such as userinfo) that someone might try to put into a URL.

The accepted answer is quite okay. But if the field (url) is optional, it may raise an error such as undefined method + for nil class. The following should resolve that:

def smart_add_url_protocol
  if self.url && !url_protocol_present?
    self.url = "http://#{self.url}"
  end
end

def url_protocol_present?
  self.url[/\Ahttp:\/\//] || self.url[/\Ahttps:\/\//]
end