ActiveRecord: How to order by weighted average of associated through model?

I have 3 models,

    #user.rb
    class User < ActiveRecord::Base
       has_many :pictures
       has_many :ratings, dependent:  :destroy
       has_many :rated_pictures, through: :ratings, source: :picture
    end

    #picture.rb    
    class Picture < ActiveRecord::Base
       #id, image, :user_id
       has_many :ratings, dependent:  :destroy
       has_many :rated_users, through: :ratings, source: :user

       def weighted_average
          data = ratings.group("stars").count
          w_sum = 0
          data.each{ |key, value| w_sum += key * value }
          (w_sum.to_f/data.values.sum).round(2)
       end
    end

    #rating.rb
    class Rating < ActiveRecord::Base
      #id, user_id, picture_id and stars
      belongs_to :user
      belongs_to :picture
    end

User can rate other user's picture out of 5, I am calculating average(weighted average) of a picture using the function picture.weighted_average.

# To can get a user's pictures 
user = User.find_by_email 'abc@sample.com'
pictures = user.pictures

My query is how to order a user pictures by weighted_average using ActiveRecord?