Very Tricky Rails Validation Problem

I am having a very tricky problem here with validation. So basically, I have a join model screen_weights with an extra attribute called "weight". The join model is for two models:score and screen.

Screen has_many :scores through=>:screen_weights

Score has_many :screens through=>:screen_weights

and then my scores are weighed depending on the weight.

let's say I have

screen_id=1,score_id=1,weight=0.3;

screen_id=1,score_id=2,weight=0.7.

In this way, I will have something as screen 1 which has two scores (1 and 2) with weights 0.3 and 0.7 respectively. The validation needs to be done is sum to 1. I will need to check for a particular screen if the weights for the scores add up to 1. How would I achieve this? SEL开发者_Python百科ECT SUM(weight) FROM screen_weights GROUP BY screen_id can give me this information. But how can I write a validation for it? Thanks a lot

class Screen
  has_many :screen_weights
  validate :weights_sum_to_1

  def weights_sum_to_1
    errors.add_to_base("Weights must add up to 1") unless screen_weights.sum(:weight) == 1
  end
end

I think this is what you're going for, though from your explanation of the problem its hard to tell exactly what the classes are doing.