Erlang Mnesia数据库

%% —
%% Excerpted from “Programming Erlang, Second Edition”,
%% published by The Pragmatic Bookshelf.
%% Copyrights apply to this code. It may not be used to create training material,
%% courses, books, articles, and the like. Contact us if you are in doubt.
%% We make no guarantees that this code is fit for any purpose.
%% Visit http://www.pragmaticprogrammer.com/titles/jaerlang2 for more book information.
%%—
-module(test_mnesia2).
-import(lists, [foreach/2]).
-compile(export_all).

%% IMPORTANT: The next line must be included
%% if we want to call qlc:q(…)

-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).

%% SQL equivalent
%% SELECT * FROM shop;

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

%% SQL equivalent
%% SELECT item, quantity FROM shop;

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

%% SQL equivalent
%% SELECT shop.item FROM shop
%% WHERE shop.quantity < 250;

demo(reorder) ->
do(qlc:q([X#shop.item || X <- mnesia:table(shop),
X#shop.quantity < 250
]));
%% SQL equivalent
%% SELECT shop.item
%% FROM shop, cost
%% WHERE shop.item = cost.name
%% AND cost.price < 2
%% AND 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() ->
[%% The shop table
{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},
%% The cost table
{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) ->
%% Nwant = Number of oranges the farmer wants to buy
F = fun() ->
%% find the number of apples
[Apple] = mnesia:read({shop,apple}),
Napples = Apple#shop.quantity,
Apple1 = Apple#shop{quantity = Napples + 2*Nwant},
%% update the database
mnesia:write(Apple1),
%% find the number of oranges
[Orange] = mnesia:read({shop,orange}),
NOranges = Orange#shop.quantity,
if
NOranges >= Nwant ->
N1 = NOranges - Nwant,
Orange1 = Orange#shop{quantity=N1},
%% update the database
mnesia:write(Orange1);
true ->
%% Oops – not enough oranges
mnesia:abort(oranges)
end
end,
mnesia:transaction(F).

farmer2(Nwant) ->
F = fun() ->
%% find the number of apples
[Apple] = mnesia:read({shop,apple}),
Napples = Apple#shop.quantity,
Apple1 = Apple#shop{quantity = Napples + 2*Nwant},
%% update the database
mnesia:write(Apple1),
%% find the number of oranges
[Orange] = mnesia:read({shop,orange}),
NOranges = Orange#shop.quantity,
N1 = NOranges - Nwant,
Orange1 = Orange#shop{quantity=N1},
mnesia:write(Orange1)
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},
{windows,12},
{rooms,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).