undefined local variable or method `current_user' devise rails

I am trying to write a method in my "team" model but current_user is showing this error

undefined local variable or method `current_user' for #

def set_default_url
  if current_user.id == self.user_id
    "/assets/default_black_:style_logo.jpg"
  else
    "/assets/default_:style_logo.jpg"
  end
end

The method current_user is working fine for other models and controllers . I am calling this method like this .

has_attached_file :logo, :styles => { 
  :medium => "200x200#", 
  :thumb => "100x100#", 
  :small => "50x50#" 
}, 
:default_url => :set_default_url

I am using rails 3.2 , ruby 1.9.3 and devise 3.1 . It seems to be simple task but I don't understand where the fault lies . I will be really thankful if someone helps me out here .