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Is there a gem to add the @ symbol on username after create the user?
Or this is just a html data on the websites like twitter?
You can always add this to the username as part of a before_validation call:
return unless (self.username? and !self.username[0,1] == '@')
self.username = '@' + self.username
Then the rest is just using it in a URL. Keep in mind @ in URLs can be problematic, it's technically not allowed, and may show up as an ugly %40.