before_save_gotcha_1.rb

before_save :do_something

def do_something
  self.attr_c = self.attr_a < self.attr_b
end

before_save_gotcha_2.rb

# this works, note that we add logging to the last line. 
def do_something
  self.attr_c = self.attr_a < self.attr_b
  logger.debug 'some message'
end

before_save_gotcha_3.rb

def do_something
  self.attr_c = self.attr_a < self.attr_b
  true
end