Fix `cache_key` with a relation having distinct and order

We can't replace existing SELECT list as long as having DISTINCT, it
will cause incorrect result.

And also, PostgreSQL has a limitation that ORDER BY expressions must
appear in select list for SELECT DISTINCT.

Therefore, we should not replace existing SELECT list when using
DISTINCT.

Fixes #29779.

2 files changed, 8 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/collection_cache_key.rb b/activerecord/lib/active_record/collection_cache_key.rb
index 12e4fae865..0520591f4f 100644
--- a/activerecord/lib/active_record/collection_cache_key.rb
+++ b/activerecord/lib/active_record/collection_cache_key.rb
@@ -6,8 +6,8 @@ module ActiveRecord
       query_signature = ActiveSupport::Digest.hexdigest(collection.to_sql)
       key = "#{collection.model_name.cache_key}/query-#{query_signature}"
 
-      if collection.loaded?
-        size = collection.size
+      if collection.loaded? || collection.distinct_value
+        size = collection.records.size
         if size > 0
           timestamp = collection.max_by(&timestamp_column)._read_attribute(timestamp_column)
         end
diff --git a/activerecord/test/cases/collection_cache_key_test.rb b/activerecord/test/cases/collection_cache_key_test.rb
index c3af32394e..cfe95b2360 100644
--- a/activerecord/test/cases/collection_cache_key_test.rb
+++ b/activerecord/test/cases/collection_cache_key_test.rb
@@ -142,6 +142,12 @@ module ActiveRecord
       assert_match(/\Adevelopers\/query-(\h+)-(\d+)-(\d+)\z/, developers.cache_key)
     end
 
+    test "cache_key with a relation having distinct and order" do
+      developers = Developer.distinct.order(:salary).limit(5)
+
+      assert_match(/\Adevelopers\/query-(\h+)-(\d+)-(\d+)\z/, developers.cache_key)
+    end
+
     test "cache_key with a relation having custom select and order" do
       developers = Developer.select("name AS dev_name").order("dev_name DESC").limit(5)