ruby on rails - can't get model to properly validate -

i have following model. works when don't fill in rep or weight keep getting error: undefined method `*' nil:nilclass

and in calculate_tonnage method.

is there way can check presence of reps or weight, calculate_tonnage en then check if tonnage unique.

i assume solution cannot find how this. have clue best way solve problem?

class submission < activerecord::base   belongs_to :user   belongs_to :contest    validates_presence_of :user_id, :reps, :weight   validates_uniqueness_of :tonnage, scope: [:user_id, :contest_id]    after_validation :calculate_tonnage    # model helpers   def calculate_tonnage     self.tonnage = self.weight * self.reps   end  end 

sure, change code to:

  def calculate_tonnage     self.tonnage = weight * reps if weight && reps   end