test_mnesia.erl

-module(test_mnesia).
-import(lists, [foreach/2]).
-compile(export_all).

-include_lib("stdlib/include/qlc.hrl").

-record(shop, {item, quantity, cost}).
-record(cost, {name, price}).
-record(design, {id, plan}).

do_this_once() ->
  mnesia:create_schema([node()]),
  mnesia:start(),
  mnesia:create_table(shop,   [{attributes, record_info(fields, shop)}]),
  mnesia:create_table(cost,   [{attributes, record_info(fields, cost)}]),
  mnesia:create_table(design, [{attributes, record_info(fields, design)}]),
  mnesia:stop().

start() ->
  mnesia:start(),
  mnesia:wait_for_tables([shop, cost, design], 20000).

demo(select_shop) ->
  do(qlc:q([X || X <- mnesia:table(shop)]));

demo(select_some) ->
  do(qlc:q([{X#shop.item, X#shop.quantity} || X <- mnesia:table(shop)]));

demo(reorder) ->
  do(qlc:q([X#shop.item || X <- mnesia:table(shop), X#shop.quantity < 250]));

demo(join) ->
  do(qlc:q([X#shop.item || X <- mnesia:table(shop), 
    X#shop.quantity < 250,
    Y <- mnesia:table(cost),
    X#shop.item =:= Y#cost.name,
    Y#cost.price < 2])).

do(Q) ->
  F = fun() -> qlc:e(Q) end,
  {atomic, Val} = mnesia:transaction(F),
  Val.

example_tables() ->
  [
  {shop, apple,  20,   2.3},
  {shop, orange, 100,  3.8},
  {shop, pear,   200,  3.6},
  {shop, banana, 420,  4.5},
  {shop, potato, 2456, 1.2},

  {cost, apple,  1.5},
  {cost, orange, 2.4},
  {cost, pear,   2.2},
  {cost, banana, 1.5},
  {cost, potato, 0.6}
  ].

add_shop_item(Name, Quantity, Cost) ->
  Row = #shop{item=Name, quantity=Quantity, cost=Cost},
  F = fun() -> mnesia:write(Row) end,
  mnesia:transaction(F).

remove_shop_item(Item) ->
  Oid = {shop, Item},
  F = fun() -> mnesia:delete(Oid) end,
  mnesia:transaction(F).

farmer(Nwant) ->
  F = fun() ->
    [Apple] = mnesia:read({shop, apple}),
    NApples = Apple#shop.quantity,
    Apple1  = Apple#shop{quantity = NApples + 2 * Nwant},
    mnesia:write(Apple1),
    [Orange] = mnesia:read({shop, orange}),
    NOranges = Orange#shop.quantity,
    if
      NOrange >= Nwant -> 
        N1 = NOrange - Nwant,
        Orange1 = Orange#shop{quantity=N1},
        mnesia:write(Orange1);
      true -> mnesia:abort(oranges)
    end
  end,
  mnesia:transaction(F).

reset_tables() ->
  mnesia:clear_table(shop),
  mnesia:clear_table(cost),
  F = fun() -> foreach(fun mnesia:write/1, example_tables()) end,
  mnesia:transaction(F).

add_plans() ->
  D1 = #design{id = {joe, 1},            plan = {circle, 10}},
  D2 = #design{id = fred,                plan = {rectangle, 10, 5}},
  D3 = #design{id = {jane, {house, 23}}, plan = {house, [
  {floor, 1, [{doors, 3}, {window, 2}, {room, 5}]}, 
  {floor, 2, [{doors, 2}, {rooms,  4}, {windows, 15}]}]}},
  F = fun() ->
    mnesia:write(D1),
    mnesia:write(D2),
    mnesia:write(D3)
  end,
  mnesia:transaction(F).

get_plan(PlanId) ->
  F = fun() -> mnesia:read({design, PlanId}) end,
  mnesia:transaction(F).