Fix issue with `cache_key` when the named timestamp column has value nil

- When the named timestamp column is nil, we should just return the
  cache_key with model name and id similar to the behavior of implicit
  timestamp columns.
- Fixed one of the issue mentioned in https://github.com/rails/rails/issues/26417.

1 files changed, 6 insertions, 0 deletions

diff --git a/activerecord/test/cases/integration_test.rb b/activerecord/test/cases/integration_test.rb
index 766917b196..00457965d7 100644
--- a/activerecord/test/cases/integration_test.rb
+++ b/activerecord/test/cases/integration_test.rb
@@ -172,4 +172,10 @@ class IntegrationTest < ActiveRecord::TestCase
     owner = owners(:blackbeard)
     assert_equal "owners/#{owner.id}-#{owner.happy_at.utc.to_s(:usec)}", owner.cache_key(:updated_at, :happy_at)
   end
+
+  def test_cache_key_when_named_timestamp_is_nil
+    owner = owners(:blackbeard)
+    owner.happy_at = nil
+    assert_equal "owners/#{owner.id}", owner.cache_key(:happy_at)
+  end
 end