Erlang SQL Migrations

This is a very simple utility to add Rails-style migrations to Erlang projects. Keep in mind that it doesn't have any database abstraction in it, you'll still operate with plain SQL. Moreso, it only supports PostgreSQL (using epgsql library) at this moment, but this is fairly easy to extend if anybody will need that kind of functionality.

Use

In order to use the tool one writes 'migration' modules. Most important thing here is to name in the order of their versioning. Our suggestion is to use [timestamp]_name.erl format. One can generate timestamps with date +%s shell command. Erlang equivalent is calendar:datetime_to_gregorian_seconds( calendar:now_to_universal_time(now()) ) - calendar:datetime_to_gregorian_seconds( {{1970,1,1},{0,0,0}} ).

-module(1323220832_add_table).
-export([upgrade/1, downgrade/1]).
-behaviour(sql_migration). %% this line is important

upgrade(C) ->
   pgsql:squery(C, "CREATE TABLE a ()").

downgrade(C) ->
   pgsql:squery(C, "DROP TABLE a").

We also suggest to put these files somewhere in priv and use this in rebar.config:

{erl_opts, [{src_dirs, ["src","priv/schema"]}]}.

This way these migration files will not interfere with your regular source code.

In your application, this is how you use sqlmig:

Migs = sql_migration:migrations(myapp),
sql_migration:migrate(Conn, hd(lists:reverse(Migs)), Migs).

You can put it in your application module (myapp_app.erl), right after supervisor startup:

start(_StartType, _StartArgs) ->
  {ok, Sup = myapp_sup:start_link(),
  init_schema(),
  {ok, Sup}.