inject_to_map.rb

module AttributesSort
  def self.included(receiver)
    receiver.instance_eval do
      def build_attributes(attributes)
        "[" + attributes.map!{|a| "object.#{a}"}.join(",") + "]"
      end

      def do_attributes_sort(collection, options={})
        attributes = build_attributes(options[:sort_by])
        collection.sort_by{|object| eval(attributes)}
      end
    end
    Array.class_eval do
      def attr_sort(options = {})
        raise "You must pass in sort_by criteria" unless options[:sort_by]
        begin
          klass = self.first.class
          klass.do_attributes_sort(self, options)
        rescue NoMethodError
          raise "You must sort by an attribute that exists in the object that you are sorting"
        end
      end
    end
  end
end


#and corresponding tests ###########################################
require File.expand_path(File.dirname(__FILE__) + '/spec_helper')

class Person
  include AttributesSort

  attr_accessor :firstname,:lastname,:age

  def initialize(firstname="",lastname="",age=0)
    @firstname = firstname
    @lastname = lastname
    @age = age
  end
end


describe "AttributesSort" do
  before(:each) do
    @p1 = Person.new("joe","blow",89)
    @p2 = Person.new("joe","blow",12)
    @p3 = Person.new("mary","watson",32)
    @p4 = Person.new("annie","watson",9)
    @p5 = Person.new("bob","builder",12)
    @people = [@p1, @p2, @p3, @p4, @p5]
  end

  def test_sort(criteria, result)
    @people.attr_sort(:sort_by => criteria).should == result
  end

  it "sort_by standard ruby" do
    @people.sort_by{|person| [person.age,person.firstname]}.should == [@p4, @p5, @p2, @p3, @p1]
  end

  it "object must have attr accessible attributes for sort_by to work" do
    p = Person.new
    p.should respond_to(:firstname)
    p.should respond_to(:lastname)
    p.should respond_to(:age)
  end

  #new test to show what build attributes is doing
  it "build attributes string array to be eval'd at deferred time in sort_by block'" do
    criteria = [:lastname, :age, :firstname]
    Person.build_attributes(criteria).should == "[object.lastname,object.age,object.firstname]"
  end

  it "sort by last name" do
    test_sort([:lastname], [@p1, @p2, @p5, @p4, @p3])
  end

  it "sort by age" do
    test_sort([:age], [@p4, @p2, @p5, @p3, @p1])
  end

  it "sort by age then by first name" do
    test_sort([:age, :firstname], [@p4, @p5, @p2, @p3, @p1])
  end

  it "sort by age then by last name" do
    test_sort([:age, :lastname], [@p4, @p2, @p5, @p3, @p1])
  end

  it "sort by last name then by first name then by age" do
    test_sort([:lastname,:firstname,:age], [@p2, @p1, @p5, @p4, @p3])
  end

  it "raise error if criteria is not provided" do
    lambda {test_sort(nil, [@p1, @p2, @p5, @p4, @p3])}.should raise_error("You must pass in sort_by criteria")
  end

  it "raise error if criteria is not a valid attribute of class" do
    lambda {test_sort([:blah], [@p1, @p2, @p5, @p4, @p3])}.should raise_error("You must sort by an attribute that exists in the object that you are sorting")
  end

end