Mnesia Add New Column In Production Environment Without Losing Data

Mnesia Utils

%%%-------------------------------------------------------------------
%%% @author aardvocate
%%% @copyright (C) 2014, <COMPANY>
%%% @doc
%%%
%%% @end
%%% Created : 03. Sep 2014 11:02 PM
%%%-------------------------------------------------------------------
%%% 
%%% This module provides a method will add new mnesia columns to an already running mnesia instance without losing data.
%%% This has only been tested on Tables with very small data. If you have a Table with millions of sensitive data,
%%%     make sure you KNOW what you are doing before runing this module. I advice you take a backup first for starters.
%%% Also this module is basic and barebones, it doesn't take into consideration for instance mnesia running on  multiple nodes,
%%%     on multiple machines. If you have erlang on multiple nodes, one solution is to run the module on each of your nodes.
%%% This module did not use LOCKING, so it is better to make sure no other process is writing to your tables while this module 
%%%     is running. Alternately, you can replace the qlc: aspects with mnesia:read/ and use locks as you wish. But also take
%%%     note that if you have 100M rows in the mnesia table, this might take a while to run. You can put this in a gen_server:cast 
%%%     so as to make it non-blocking.


-module(mnesia_utils).
-author("segun").

%% API
-export([start/1, add_column/4]).
-include_lib("stdlib/include/qlc.hrl").

start(MnesiaDir) ->
  application:set_env(mnesia, dir, MnesiaDir),
  mnesia:start().
  
add_column(TableName, ExistingColumns, NewColum, NewColumnDefaulValue) ->
  ReadFun = fun() ->
      Query = qlc:q([R || R <- mnesia:table(TableName)]),
    qlc:e(Query)
  end,
  {atomic, Records} = mnesia:transaction(ReadFun),
  mnesia:delete_table(TableName),
  Columns = lists:reverse([NewColum | lists:reverse(ExistingColumns)]),
  mnesia:create_table(TableName, [{attributes, Columns}]),
  MnesiaData = generate_mnesia_data(Records, NewColumnDefaulValue),
  F = fun() -> lists:foreach(fun mnesia:write/1, MnesiaData) end,
  mnesia:transaction(F).

prepend(X, Tuple) ->
  TupleAsList = lists:reverse(tuple_to_list(Tuple)),
  list_to_tuple(lists:reverse([X | TupleAsList])).

generate_mnesia_data(Records, Default) ->
  generate_mnesia_data(Records, Default, []).

generate_mnesia_data([H | Records], Default, Acc) ->
  NewRecord = prepend(Default, H),
  generate_mnesia_data(Records, Default, [NewRecord | Acc]);

generate_mnesia_data([], _Default, Acc) ->
  lists:reverse(Acc).

Did you ever consider using mnesia:transform_table/3 ?

http://erlang.org/doc/man/mnesia.html#transform_table-3