weight.rb (RoR): Lets you pick a row from a table randomly based on the rows weight.

weight.rb

#
#
# weight.rb (A Ruby on Rails Model).
# Lets you pick a row from a table randomly based on the rows weight.
#
# Your table will need three columns:
#  - weight       (float)
#  - weight_begin (float)
#  - weight_end   (float)
#
# You can also support multiple categories by adding a string column to your
# table, and setting @@category_column to the columns name. Each category
# will have it's own weight scope.
#
# The code within the class...end definition can be pasted into your own
# Model without any modification cause I'm using "self.class.to_s.constantize"
# instead of "Weight" within the instance methods.
#
# The weight_begin and weight_end "index" columns are auto-updated when
# you call the save method on new or existing row objects.
#
#

class Weight < ActiveRecord::Base

  after_create :update_weights
  after_update :update_weights
  after_destroy :update_weights
  @@category_column = nil
  
  def self.pick(category = nil)
    scope = rand(10000).to_f / 100.00
    if category.nil?
      conditions = ["weight_begin <= ? AND weight_end >= ?", scope, scope]
    else
      conditions = ["weight_begin <= ? AND weight_end >= ? AND #{@@category_column} = ?", scope, scope, category]
    end
    self.find(:first, :conditions => conditions)
  end
  
  def self.reset_weights    
    self.update_all({:weight_begin => nil, :weight_end => nil})
  end
  
  def self.update_weights
    self.new.update_weights
  end
  
  def update_weights
    self_class = self.class.to_s.constantize
    category_changed = self.send("#{@@category_column}_changed?") || false
    if self.new_record? || self.weight_changed? || !category_changed.nil?
      categories = [nil]
      if !@@category_column.nil? && self_class.column_names.include?(@@category_column.to_s)
        categories = self_class.find(:all, :select => "distinct #{@@category_column}").map { |cat| cat.send(@@category_column) }
      end
      categories.each do |category|
        conditions = (!category.nil?) ? {@@category_column.to_sym => category} : {}
        total = self_class.sum(:weight, :conditions => conditions)
        rows = self_class.find(:all, :conditions => conditions)
        previous = 0.00
        size = rows.size
        rows.each_with_index do |row, i|
          previous += 0.01 if previous > 0.00
          weight_begin = previous
          weight_end = ((previous + (row.weight.to_f / total.to_f * 100))).round(2)
          weight_end = 100.00 if i == (size-1)
          previous = weight_end
          self_class.update_all({:weight_begin => weight_begin, :weight_end => weight_end}, {:id => row.id})
        end
      end      
    end
    return nil
  end

end