Fix MySQL schema dump for `timestamp`

config/initializers/active_record_data_types.rb

@@ -45,11 +45,37 @@ if Gitlab::Database.postgresql?
 elsif Gitlab::Database.mysql?
   require 'active_record/connection_adapters/mysql2_adapter'
 
-  module ActiveRecord
-    module ConnectionAdapters
-      class AbstractMysqlAdapter
-        NATIVE_DATABASE_TYPES[:datetime_with_timezone] = { name: 'timestamp' }
+  module RegisterDateTimeWithTimeZone
+    # Run original `initialize_type_map` and then register `timestamp` as a
+    # `MysqlDateTimeWithTimeZone`.
+    #
+    # When schema dumping, `timestamp` columns will be output as
+    # `t.datetime_with_timezone`.
+    def initialize_type_map(mapping)
+      super mapping
+
+      mapping.register_type(%r(timestamp)i) do |sql_type|
+        precision = extract_precision(sql_type)
+        ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter::MysqlDateTimeWithTimeZone.new(precision: precision)
+      end
     end
   end
+
+  class ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter
+    prepend RegisterDateTimeWithTimeZone
+
+    # Add the class `DateTimeWithTimeZone` so we can map `timestamp` to it.
+    class MysqlDateTimeWithTimeZone < MysqlDateTime
+      def type
+        :datetime_with_timezone
+      end
+    end
+
+    # Add column type `datetime_with_timezone` so we can do this in
+    # migrations:
+    #
+    #   add_column(:users, :datetime_with_timezone)
+    #
+    NATIVE_DATABASE_TYPES[:datetime_with_timezone] = { name: 'timestamp' }
   end
 end