From 9eba4126c8c110b7d67316a3e7f117be4b9df35a Mon Sep 17 00:00:00 2001
From: Ryuta Kamizono <kamipo@gmail.com>
Date: Fri, 19 Jul 2019 23:01:44 +0900
Subject: [PATCH] Merge pull request #36706 from kirs/dedup-optimizer-hints

[ActiveRecord] Deduplicate optimizer hints
---
 activerecord/lib/active_record/relation/query_methods.rb | 2 +-
 activerecord/test/cases/relation_test.rb                 | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/activerecord/lib/active_record/relation/query_methods.rb b/activerecord/lib/active_record/relation/query_methods.rb
index 87a53966e4..6a181882ae 100644
--- a/activerecord/lib/active_record/relation/query_methods.rb
+++ b/activerecord/lib/active_record/relation/query_methods.rb
@@ -952,7 +952,7 @@ def optimizer_hints(*args)
     def optimizer_hints!(*args) # :nodoc:
       args.flatten!
 
-      self.optimizer_hints_values += args
+      self.optimizer_hints_values |= args
       self
     end
 
diff --git a/activerecord/test/cases/relation_test.rb b/activerecord/test/cases/relation_test.rb
index 3f370e5ede..4b2229e8db 100644
--- a/activerecord/test/cases/relation_test.rb
+++ b/activerecord/test/cases/relation_test.rb
@@ -363,6 +363,13 @@ def test_relation_with_optimizer_hints_filters_sql_comment_delimiters
       assert_match %r{/\*\+ BADHINT \*/}, post_with_hint.to_sql
     end
 
+    def test_does_not_duplicate_optimizer_hints_on_merge
+      escaped_table = Post.connection.quote_table_name("posts")
+      expected = "SELECT /*+ OMGHINT */ #{escaped_table}.* FROM #{escaped_table}"
+      query = Post.optimizer_hints("OMGHINT").merge(Post.optimizer_hints("OMGHINT")).to_sql
+      assert_equal expected, query
+    end
+
     class EnsureRoundTripTypeCasting < ActiveRecord::Type::Value
       def type
         :string
-- 
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