Allow users to choose the timestamp format in the cache key

This can be done using the class attribute cache_timestamp_format

Closes #8195

4 files changed, 34 insertions, 2 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index f12094ab1e..e8ae999b33 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,12 @@
 ## Rails 3.2.10 (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*
+
 *   Unscope `update_column(s)` query to ignore default scope.
 
     When applying `default_scope` to a class with a where clause, using
diff --git a/activerecord/lib/active_record/integration.rb b/activerecord/lib/active_record/integration.rb
index 23c272ef12..f2ace18d44 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 = :number
+    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.
@@ -39,7 +50,7 @@ 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 4e0f8054c7..2497b18e15 100644
--- a/activerecord/test/cases/base_test.rb
+++ b/activerecord/test/cases/base_test.rb
@@ -2153,10 +2153,17 @@ class BasicsTest < ActiveRecord::TestCase
 
   def test_cache_key_format_for_existing_record_with_updated_at
     dev = Developer.first
-    assert_equal "developers/#{dev.id}-#{dev.updated_at.utc.to_s(:nsec)}", dev.cache_key
+    assert_equal "developers/#{dev.id}-#{dev.updated_at.utc.to_s(:number)}", dev.cache_key
+  end
+
+  def test_cache_key_format_for_existing_record_with_updated_at_1
+    dev = CachedDeveloper.first
+    assert_equal "cached_developers/#{dev.id}-#{dev.updated_at.utc.to_s(:nsec)}", dev.cache_key
   end
 
   def test_cache_key_changes_when_child_touched
+    old_timestamp_format = Car.cache_timestamp_format
+    Car.cache_timestamp_format = :nsec
     car = Car.create
     Bulb.create(car: car)
 
@@ -2164,6 +2171,8 @@ class BasicsTest < ActiveRecord::TestCase
     car.bulb.touch
     car.reload
     assert_not_equal key, car.cache_key
+  ensure
+    Car.cache_timestamp_format = old_timestamp_format
   end
 
   def test_cache_key_format_for_existing_record_with_nil_updated_at
diff --git a/activerecord/test/models/developer.rb b/activerecord/test/models/developer.rb
index a730d86586..f8b758b3c2 100644
--- a/activerecord/test/models/developer.rb
+++ b/activerecord/test/models/developer.rb
@@ -245,3 +245,8 @@ class ThreadsafeDeveloper < ActiveRecord::Base
     limit(1)
   end
 end
+
+class CachedDeveloper < ActiveRecord::Base
+  self.table_name = "developers"
+  self.cache_timestamp_format = :nsec
+end