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, 36 insertions, 15 deletions

diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index cd62b0b881..8eb71e6454 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -291,27 +291,23 @@ module ActiveRecord
       limit_value ? records.many? : size > 1
     end
 
-    # Returns a cache key that can be used to identify the records fetched by
-    # this query. The cache key is built with a fingerprint of the sql query,
-    # the number of records matched by the query and a timestamp of the last
-    # updated record. When a new record comes to match the query, or any of
-    # the existing records is updated or deleted, the cache key changes.
+    # Returns a stable cache key that can be used to identify this query.
+    # The cache key is built with a fingerprint of the SQL query.
     #
-    #   Product.where("name like ?", "%Cosmic Encounter%").cache_key
-    #   # => "products/query-1850ab3d302391b85b8693e941286659-1-20150714212553907087000"
+    #    Product.where("name like ?", "%Cosmic Encounter%").cache_key
+    #    # => "products/query-1850ab3d302391b85b8693e941286659"
     #
-    # If the collection is loaded, the method will iterate through the records
-    # to generate the timestamp, otherwise it will trigger one SQL query like:
+    # If ActiveRecord::Base.collection_cache_versioning is turned off, as it was
+    # in Rails 6.0 and earlier, the cache key will also include a version.
     #
-    #    SELECT COUNT(*), MAX("products"."updated_at") FROM "products" WHERE (name like '%Cosmic Encounter%')
+    #    ActiveRecord::Base.collection_cache_versioning = false
+    #    Product.where("name like ?", "%Cosmic Encounter%").cache_key
+    #    # => "products/query-1850ab3d302391b85b8693e941286659-1-20150714212553907087000"
     #
     # You can also pass a custom timestamp column to fetch the timestamp of the
     # last updated record.
     #
     #   Product.where("name like ?", "%Game%").cache_key(:last_reviewed_at)
-    #
-    # You can customize the strategy to generate the key on a per model basis
-    # overriding ActiveRecord::Base#collection_cache_key.
     def cache_key(timestamp_column = :updated_at)
       @cache_keys ||= {}
       @cache_keys[timestamp_column] ||= @klass.collection_cache_key(self, timestamp_column)
@@ -321,6 +317,31 @@ module ActiveRecord
       query_signature = ActiveSupport::Digest.hexdigest(to_sql)
       key = "#{klass.model_name.cache_key}/query-#{query_signature}"
 
+      if cache_version(timestamp_column)
+        key
+      else
+        "#{key}-#{compute_cache_version(timestamp_column)}"
+      end
+    end
+
+    # Returns a cache version that can be used together with the cache key to form
+    # a recyclable caching scheme. The cache version is built with the number of records
+    # matching the query, and the timestamp of the last updated record. When a new record
+    # comes to match the query, or any of the existing records is updated or deleted,
+    # the cache version changes.
+    #
+    # If the collection is loaded, the method will iterate through the records
+    # to generate the timestamp, otherwise it will trigger one SQL query like:
+    #
+    #    SELECT COUNT(*), MAX("products"."updated_at") FROM "products" WHERE (name like '%Cosmic Encounter%')
+    def cache_version(timestamp_column = :updated_at)
+      if collection_cache_versioning
+        @cache_versions ||= {}
+        @cache_versions[timestamp_column] ||= compute_cache_version(timestamp_column)
+      end
+    end
+
+    def compute_cache_version(timestamp_column) # :nodoc:
       if loaded? || distinct_value
         size = records.size
         if size > 0
@@ -356,9 +377,9 @@ module ActiveRecord
       end
 
       if timestamp
-        "#{key}-#{size}-#{timestamp.utc.to_s(cache_timestamp_format)}"
+        "#{size}-#{timestamp.utc.to_s(cache_timestamp_format)}"
       else
-        "#{key}-#{size}"
+        "#{size}"
       end
     end