caching last in AR

last.rb

Role.count
   (0.5ms)  SELECT COUNT(*) FROM "roles" 
 => 0 
1.8.7 :037 > Role.last
  Role Load (0.5ms)  SELECT "roles".* FROM "roles" ORDER BY "roles"."id" DESC LIMIT 1
 => nil 
1.8.7 :038 > Role.create!(:name => 'php_committer')
   (0.2ms)  BEGIN
  Role Exists (0.4ms)  SELECT 1 FROM "roles" WHERE "roles"."name" = 'php_committer' LIMIT 1
  SQL (0.7ms)  INSERT INTO "roles" ("created_at", "description", "name", "title", "updated_at") VALUES ($1, $2, $3, $4, $5) RETURNING "id"  [["created_at", Fri, 01 Jun 2012 01:15:04 MSK +04:00], ["description", nil], ["name", "php_committer"], ["title", nil], ["updated_at", Fri, 01 Jun 2012 01:15:04 MSK +04:00]]
   (0.6ms)  COMMIT
 => #<Role id: 15, name: "php_committer", created_at: "2012-05-31 21:15:04", updated_at: "2012-05-31 21:15:04", title: nil, description: nil> 
1.8.7 :039 > Role.last
  Role Load (0.5ms)  SELECT "roles".* FROM "roles" ORDER BY "roles"."id" DESC LIMIT 1
 => #<Role id: 15, name: "php_committer", created_at: "2012-05-31 21:15:04", updated_at: "2012-05-31 21:15:04", title: nil, description: nil> 
1.8.7 :040 > Role.create!(:name => 'ruby_committer')
   (0.2ms)  BEGIN
  Role Exists (0.4ms)  SELECT 1 FROM "roles" WHERE "roles"."name" = 'ruby_committer' LIMIT 1
  SQL (0.6ms)  INSERT INTO "roles" ("created_at", "description", "name", "title", "updated_at") VALUES ($1, $2, $3, $4, $5) RETURNING "id"  [["created_at", Fri, 01 Jun 2012 01:15:12 MSK +04:00], ["description", nil], ["name", "ruby_committer"], ["title", nil], ["updated_at", Fri, 01 Jun 2012 01:15:12 MSK +04:00]]
   (0.6ms)  COMMIT
 => #<Role id: 16, name: "ruby_committer", created_at: "2012-05-31 21:15:12", updated_at: "2012-05-31 21:15:12", title: nil, description: nil> 
1.8.7 :041 > Role.last
 => #<Role id: 15, name: "php_committer", created_at: "2012-05-31 21:15:04", updated_at: "2012-05-31 21:15:04", title: nil, description: nil>