ruby on rails 3 - paper trail gem - undefined local variable or method has_paper_trail -

i using latest version of paper_trail gem (2.7.1) after gem installed , table created have added "has_paper_trail" in model , end getting "undefined local variable or method has_paper_trail". tried lower versions still getting same error.

this silly mistake need restart server after gem installation. newbies me makes such mistakes. full credit thomas kind information.