use Module#prepend instead of alias_method_chain

config/initializers/mysql_set_length_for_binary_indexes.rb

@@ -2,12 +2,7 @@
 # MySQL adapter apply a length of 20. Otherwise MySQL can't create an index on
 # binary columns.
 
-if defined?(ActiveRecord::ConnectionAdapters::Mysql2Adapter)
-  module ActiveRecord
-    module ConnectionAdapters
-      class Mysql2Adapter < AbstractMysqlAdapter
-        alias_method :__gitlab_add_index2, :add_index
-
+module MysqlSetLengthForBinaryIndex
   def add_index(table_name, column_names, options = {})
     Array(column_names).each do |column_name|
       column = ActiveRecord::Base.connection.columns(table_name).find { |c| c.name == column_name }
@@ -17,9 +12,10 @@ if defined?(ActiveRecord::ConnectionAdapters::Mysql2Adapter)
       end
     end
 
-          __gitlab_add_index2(table_name, column_names, options)
-        end
-      end
-    end
+    super(table_name, column_names, options)
   end
 end
+
+if defined?(ActiveRecord::ConnectionAdapters::Mysql2Adapter)
+  ActiveRecord::ConnectionAdapters::Mysql2Adapter.send(:prepend, MysqlSetLengthForBinaryIndex)
+end