Extend ActiveRecord::Base#cache_key to take an optional list of timestamp attributes of which the highest will be used.

Author: David Heinemeier Hansson

activerecord/CHANGELOG.md

@@ -1,3 +1,13 @@
+*   Extend ActiveRecord::Base#cache_key to take an optional list of timestamp attributes of which the highest will be used.
+
+	Example:
+	
+		# last_reviewed_at will be used, if that's more recent than updated_at, or vice versa
+		Person.find(5).cache_key(:updated_at, :last_reviewed_at)
+
+	*DHH*
+
+
 *   Added ActiveRecord::Base#enum for declaring enum attributes where the values map to integers in the database, but can be queried by name.
 
     Example:

activerecord/lib/active_record/integration.rb

@@ -45,10 +45,18 @@ def to_param
     #   Product.new.cache_key     # => "products/new"
     #   Product.find(5).cache_key # => "products/5" (updated_at not available)
     #   Person.find(5).cache_key  # => "people/5-20071224150000" (updated_at available)
-    def cache_key
+    # 
+    # You can also pass a list of named timestamps, and the newest in the list will be
+    # used to generate the key:
+    #
+    #   Person.find(5).cache_key(:updated_at, :last_reviewed_at)
+    def cache_key(*timestamp_names)
       case
       when new_record?
         "#{self.class.model_name.cache_key}/new"
+      when timestamp_names.any?
+        timestamps = timestamp_names.collect { |method| send(method) }.compact
+        "#{self.class.model_name.cache_key}/#{id}-#{timestamps.max.utc.to_s(:number)}"        
       when timestamp = max_updated_column_timestamp
         timestamp = timestamp.utc.to_s(cache_timestamp_format)
         "#{self.class.model_name.cache_key}/#{id}-#{timestamp}"

activerecord/test/cases/integration_test.rb

@@ -3,9 +3,10 @@
 require 'models/developer'
 require 'models/car'
 require 'models/bulb'
+require 'models/owner'
 
 class IntegrationTest < ActiveRecord::TestCase
-  fixtures :companies, :developers
+  fixtures :companies, :developers, :owners
 
   def test_to_param_should_return_string
     assert_kind_of String, Client.first.to_param
@@ -81,4 +82,9 @@ def test_cache_key_format_is_precise_enough
     dev.touch
     assert_not_equal key, dev.cache_key
   end
+  
+  def test_named_timestamps_for_cache_key
+    owner = owners(:blackbeard)
+    assert_equal "owners/#{owner.id}-#{owner.happy_at.utc.to_s(:number)}", owner.cache_key(:updated_at, :happy_at)
+  end
 end

activerecord/test/fixtures/owners.yml

@@ -2,6 +2,7 @@ blackbeard:
   owner_id: 1
   name: blackbeard
   essay_id: A Modest Proposal
+  happy_at: '2150-10-10 16:00:00'
 
 ashley:
   owner_id: 2