use Module#prepend instead of alias_method_chain

config/initializers/mysql_set_length_for_binary_indexes.rb

@@ -2,24 +2,20 @@
 # 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
-
-        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 }
+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 }
 
-            if column&.type == :binary
-              options[:length] = 20
-            end
-          end
-
-          __gitlab_add_index2(table_name, column_names, options)
-        end
+      if column&.type == :binary
+        options[:length] = 20
       end
     end
+
+    super(table_name, column_names, options)
   end
 end
+
+if defined?(ActiveRecord::ConnectionAdapters::Mysql2Adapter)
+  ActiveRecord::ConnectionAdapters::Mysql2Adapter.send(:prepend, MysqlSetLengthForBinaryIndex)
+end