fixes #21815

The default timestamp used for AR is `updated_at` in nanoseconds! (:nsec) This causes issues on any machine that runs an OS that supports nanoseconds timestamps, i.e. not-OS X, where the cache_key of the record persisted in the database (milliseconds precision) is out-of-sync with the cache_key in the ruby VM.

This commit adds:

A test that shows the issue, it can be found in the separate file `cache_key_test.rb`, because
- model couldn't be defined inline
- transactional testing needed to be turned off to get it to pass the MySQL tests
This seemed cleaner than putting it in an existing testcase file.

It adds :usec as a dateformat that calculates datetime in microseconds

It sets precision of cache_key to :usec instead of :nsec, as no db supports nsec precision on timestamps

1 files changed, 2 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/integration.rb b/activerecord/lib/active_record/integration.rb
index 19d2589db3..466c8509a4 100644
--- a/activerecord/lib/active_record/integration.rb
+++ b/activerecord/lib/active_record/integration.rb
@@ -10,9 +10,9 @@ module ActiveRecord
       # Indicates the format used to generate the timestamp in the cache key.
       # Accepts any of the symbols in <tt>Time::DATE_FORMATS</tt>.
       #
-      # This is +:nsec+, by default.
+      # This is +:usec+, by default.
       class_attribute :cache_timestamp_format, :instance_writer => false
-      self.cache_timestamp_format = :nsec
+      self.cache_timestamp_format = :usec
     end
 
     # Returns a String, which Action Pack uses for constructing a URL to this