From bc021203929b8d0d241061b253a4a636997abe62 Mon Sep 17 00:00:00 2001
From: Ryuta Kamizono <kamipo@gmail.com>
Date: Tue, 17 Dec 2019 16:19:21 +0900
Subject: [PATCH] Do not allow mutation for empty clauses

---
 activerecord/lib/active_record/relation/from_clause.rb  | 2 +-
 activerecord/lib/active_record/relation/where_clause.rb | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/activerecord/lib/active_record/relation/from_clause.rb b/activerecord/lib/active_record/relation/from_clause.rb
index c53a682aee..93b129ce99 100644
--- a/activerecord/lib/active_record/relation/from_clause.rb
+++ b/activerecord/lib/active_record/relation/from_clause.rb
@@ -19,7 +19,7 @@ module ActiveRecord
       end
 
       def self.empty
-        @empty ||= new(nil, nil)
+        @empty ||= new(nil, nil).freeze
       end
     end
   end
diff --git a/activerecord/lib/active_record/relation/where_clause.rb b/activerecord/lib/active_record/relation/where_clause.rb
index 2e6f7cd3be..1bf5cacca3 100644
--- a/activerecord/lib/active_record/relation/where_clause.rb
+++ b/activerecord/lib/active_record/relation/where_clause.rb
@@ -83,7 +83,7 @@ module ActiveRecord
       end
 
       def self.empty
-        @empty ||= new([])
+        @empty ||= new([]).tap(&:referenced_columns).freeze
       end
 
       def contradiction?