Fix ActiveRecord::Relation#cache_key for loaded empty collection

- Before this patch if we try to find cache_key of a loaded but empty
  collection it used to give error because of trying to call `updated_at`
  on `nil` value generated by
  `collection.max_by(&timestamp_column).public_send(timestamp_column)`.
- This commit fixes above error by checking if size is greater than zero
  or not.

1 files changed, 3 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/collection_cache_key.rb b/activerecord/lib/active_record/collection_cache_key.rb
index 3c4ca3d116..b0e555038e 100644
--- a/activerecord/lib/active_record/collection_cache_key.rb
+++ b/activerecord/lib/active_record/collection_cache_key.rb
@@ -7,7 +7,9 @@ module ActiveRecord
 
       if collection.loaded?
         size = collection.size
-        timestamp = collection.max_by(&timestamp_column).public_send(timestamp_column)
+        if size > 0
+          timestamp = collection.max_by(&timestamp_column).public_send(timestamp_column)
+        end
       else
         column_type = type_for_attribute(timestamp_column.to_s)
         column = "#{connection.quote_table_name(collection.table_name)}.#{connection.quote_column_name(timestamp_column)}"