[Rails] How to use a parameter with a string?

[softwareengineer 99]

Hello,
  
  I am learning Ruby on Rails and have a very basic question.
  
      def self.home_categories (portal_id) 
          find(:all, 
               :conditions => "portal_id=:portal_id"
          )
      end
  
  How can I put the value of portal_id in the string "portal_id=...". I  tried concatenation but I get the error that it cannot convert.
  
  I can figure it out eventually  but thought someone may have the answer ready.
  
  Thanks
  Frank
  
		
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