From 6a9301dcc57460b7d4fb1681e82529264cc2b9c4 Mon Sep 17 00:00:00 2001
From: Ryuta Kamizono <kamipo@gmail.com>
Date: Tue, 30 Jun 2020 22:51:17 +0900
Subject: [PATCH] Backward compatibility to work
 `Marshal.load(legacy_record.dump)` for MySQL

Context https://github.com/rails/rails/pull/39599#discussion_r445429365.

Similar with #39611.

Actually we never said we would keep Mashal compatibility between two
versions of Rails, so we sometimes directly removed the dedicated types
in the past (e.g. f1a0fa9, #29666), but that direct removing makes cache
rotation harder.

Keeping the unused constant in new version at least during one version
will help cache rotation easier.
---
 .../abstract_mysql_adapter.rb                    |   4 ++++
 activerecord/test/cases/base_test.rb             |  15 +++++++++++++++
 .../legacy_6_0_record_mysql.dump                 | Bin 0 -> 1901 bytes
 3 files changed, 19 insertions(+)
 create mode 100644 activerecord/test/support/marshal_compatibility_fixtures/legacy_6_0_record_mysql.dump

diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index 9dbed2c1b7..39af3a02cf 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -844,6 +844,10 @@ def version_string(full_version_string)
           full_version_string.match(/^(?:5\.5\.5-)?(\d+\.\d+\.\d+)/)[1]
         end
 
+        # Alias MysqlString to work Mashal.load(File.read("legacy_record.dump")).
+        # TODO: Remove the constant alias once Rails 6.1 has released.
+        MysqlString = Type::String # :nodoc:
+
         ActiveRecord::Type.register(:immutable_string, adapter: :mysql2) do |_, **args|
           Type::ImmutableString.new(true: "1", false: "0", **args)
         end
diff --git a/activerecord/test/cases/base_test.rb b/activerecord/test/cases/base_test.rb
index f8f3e86acc..5e948224d7 100644
--- a/activerecord/test/cases/base_test.rb
+++ b/activerecord/test/cases/base_test.rb
@@ -1226,6 +1226,21 @@ def test_marshalling_with_associations
     assert_equal 1, post.comments.length
   end
 
+  if current_adapter?(:Mysql2Adapter)
+    def test_marshal_load_legacy_6_0_record_mysql
+      path = File.expand_path(
+        "support/marshal_compatibility_fixtures/legacy_6_0_record_mysql.dump",
+        TEST_ROOT
+      )
+      topic = Marshal.load(File.read(path))
+
+      assert_not_predicate topic, :new_record?
+      assert_equal 1, topic.id
+      assert_equal "The First Topic", topic.title
+      assert_equal "Have a nice day", topic.content
+    end
+  end
+
   if Process.respond_to?(:fork) && !in_memory_db?
     def test_marshal_between_processes
       # Define a new model to ensure there are no caches
diff --git a/activerecord/test/support/marshal_compatibility_fixtures/legacy_6_0_record_mysql.dump b/activerecord/test/support/marshal_compatibility_fixtures/legacy_6_0_record_mysql.dump
new file mode 100644
index 0000000000000000000000000000000000000000..72ebbd88d0ea24dcfa68b94ea4cbd865cc1dd699
GIT binary patch
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-- 
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