Add collection cache versioning

Cache versioning enables the same cache key to be reused when the object
being cached changes by moving the volatile part of the cache key out of
the cache key and into a version that is embedded in the cache entry.

This is already occurring when the object being cached is an
`ActiveRecord::Base`, but when caching an `ActiveRecord::Relation`
we are currently still putting the volatile information (max updated at
and count) as part of the cache key.

This PR moves the volatile part of the relations `cache_key` into the
`cache_version` to support recycling cache keys for
`ActiveRecord::Relation`s.

1 files changed, 11 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 8daa6c0ce5..59669ca26e 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,14 @@
+*   Add `ActiveRecord::Relation#cache_version` to support recyclable cache keys via
+    the versioned entries in `ActiveSupport::Cache`. This also means that
+    `ActiveRecord::Relation#cache_key` will now return a stable key that does not
+    include the max timestamp or count any more.
+
+    NOTE: This feature is turned off by default, and `cache_key` will still return
+    cache keys with timestamps until you set `ActiveRecord::Base.collection_cache_versioning = true`.
+    That's the setting for all new apps on Rails 6.0+
+
+    *Lachlan Sylvester*
+
 *   Fix dirty tracking for `touch` to track saved changes.
 
     Fixes #33429.