Allow users to choose the timestamp format in the cache key

This can be done using the class attribute cache_timestamp_format

Conflicts:
	railties/guides/source/configuring.textile

5 files changed, 31 insertions, 1 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 02aced31c9..d4e3a33002 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,12 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Add `ActiveRecord::Base.cache_timestamp_format` class attribute to control
+    the format of the timestamp value in the cache key.
+    This allows users to improve the precision of the cache key.
+    Fixes #8195.
+
+    *Rafael Mendonça França*
+
 *   Fix decorating columns for serialized attributes. Fixes #8441
 
     *itzki*
diff --git a/activerecord/lib/active_record/integration.rb b/activerecord/lib/active_record/integration.rb
index 7bdc1bd4c6..7f877a6471 100644
--- a/activerecord/lib/active_record/integration.rb
+++ b/activerecord/lib/active_record/integration.rb
@@ -1,5 +1,16 @@
 module ActiveRecord
   module Integration
+    extend ActiveSupport::Concern
+
+    included do
+      ##
+      # :singleton-method:
+      # Indicates the format used to generate the timestamp format in the cache key.
+      # This is +:number+, by default.
+      class_attribute :cache_timestamp_format, :instance_writer => false
+      self.cache_timestamp_format = :nsec
+    end
+
     # Returns a String, which Action Pack uses for constructing an URL to this
     # object. The default implementation returns this record's id as a String,
     # or nil if this record's unsaved.
@@ -37,7 +48,7 @@ module ActiveRecord
       when new_record?
         "#{self.class.model_name.cache_key}/new"
       when timestamp = self[:updated_at]
-        timestamp = timestamp.utc.to_s(:nsec)
+        timestamp = timestamp.utc.to_s(cache_timestamp_format)
         "#{self.class.model_name.cache_key}/#{id}-#{timestamp}"
       else
         "#{self.class.model_name.cache_key}/#{id}"
diff --git a/activerecord/test/cases/base_test.rb b/activerecord/test/cases/base_test.rb
index e4445698dc..c5c500279e 100644
--- a/activerecord/test/cases/base_test.rb
+++ b/activerecord/test/cases/base_test.rb
@@ -1444,6 +1444,11 @@ class BasicsTest < ActiveRecord::TestCase
     assert_equal "developers/#{dev.id}-#{dev.updated_at.utc.to_s(:nsec)}", dev.cache_key
   end
 
+  def test_cache_key_format_for_existing_record_with_updated_at_and_cache_timestamp_format_set
+    dev = CachedDeveloper.first
+    assert_equal "cached_developers/#{dev.id}-#{dev.updated_at.utc.to_s(:number)}", dev.cache_key
+  end
+
   def test_cache_key_changes_when_child_touched
     car = Car.create
     Bulb.create(car: car)
diff --git a/activerecord/test/models/developer.rb b/activerecord/test/models/developer.rb
index 1c048bb6b4..683cb54a10 100644
--- a/activerecord/test/models/developer.rb
+++ b/activerecord/test/models/developer.rb
@@ -234,3 +234,8 @@ class ThreadsafeDeveloper < ActiveRecord::Base
     limit(1)
   end
 end
+
+class CachedDeveloper < ActiveRecord::Base
+  self.table_name = "developers"
+  self.cache_timestamp_format = :number
+end
diff --git a/guides/source/configuring.md b/guides/source/configuring.md
index 446f767f0c..a8e33a2956 100644
--- a/guides/source/configuring.md
+++ b/guides/source/configuring.md
@@ -276,6 +276,8 @@ config.middleware.delete ActionDispatch::BestStandardsSupport
 
 * `config.active_record.auto_explain_threshold_in_seconds` configures the threshold for automatic EXPLAINs (`nil` disables this feature). Queries exceeding the threshold get their query plan logged. Default is 0.5 in development mode.
 
+* +config.active_record.cache_timestamp_format+ controls the format of the timestamp value in the cache key. Default is +:number+.
+
 The MySQL adapter adds one additional configuration option:
 
 * `ActiveRecord::ConnectionAdapters::MysqlAdapter.emulate_booleans` controls whether Active Record will consider all `tinyint(1)` columns in a MySQL database to be booleans and is true by default.