Ruby on Rails scope filtering

filtered_scope.rb

# Model:
#
# class Example < ActiveRecord::Base
# end
#
# Controller:
#
# class ExamplesController < ActionController::Base
#   def index
#     @examples = Example.filtered_scope(params[:filter])
#   end
# end
#
# Query String Usage:
#
# /examples?filter[title]=value&filter[min_created_at]=2011-01-01
#
# Matches only examples with 
# title equal to "value" AND 
# created_at greater than or equal to 2011-01-01

class ActiveRecord::Base
  def self.filtered_scope(filter)
    filtered_scope = self.scoped
    if filter
      for column in filter.keys
        if self.column_names.include?(column)
          filtered_scope = filtered_scope.where(column => filter[column])
        else
          name = column[4..column.length]
          if self.column_names.include?(name)
            if column[0..3]=='min_'
              filtered_scope = filtered_scope.where(["#{name} >= ?",filter[column]])
            elsif column[0..3]=='max_'
              filtered_scope = filtered_scope.where(["#{name} <= ?",filter[column]])
            end
          end
        end
      end
    end
    filtered_scope
  end
end