How to write database specific custom validator in Rails 3.0?

I need to write a custom validator to check if a record exists in the database or not. Sort of like the opposite of validate uniqueness, however I couldn't find something that could achieve what I wanted in the built in validators.

What I'm attempting to do is check if the referrer exists or not in the User开发者_高级运维s table. If the referrer's username is "testuser", I want to check in the Users table whether "testuser" actually exists.

I have created a custom validator:

class ReferrerExistsValidator < ActiveModel::EachValidator

However I'm unsure how to proceed to fetch details from there database, any pointers?

Write the following validation class

 class ReferrerExistsValidator < ActiveModel::EachValidator  
   def validate_each(object, attribute, value)  
     unless User.find_by_username(value)  
       object.errors[attribute] << (options[:message] || "referrer does not exist")  
     end  
   end  
 end 

Add the following to the relevant model

validates :referrer_exists => true

I'm new to rails/coding, and I may misunderstand you, but couldnt you just do something like:

@user.each do |user|
  unless @user.username == current_user.referral_name
    :alert "no user found"
  else whatever
  end
end

EDIT: that wouldn't work at all - ignore that, let me think about it for a sec ;)