如何在Erlang中更新Mnesia表

Question

我的代码有点问题.我有一个包含汽车详细信息,名称,价格和数量的表格,因此我正在尝试创建一个buy用于购买特定汽车的功能.当用户购买5辆宝马汽车时,他们会打电话buy_car(bmw,5).在此之后,我想更新宝马汽车的新数量值.

我的尝试在下面,但我似乎无法解决它,我是Erlang的新手.

buy_car(X,Ncars) ->

    F = fun() ->

        %% ----first i find the number of car X available in the shop
        [Xcars] = mnesia:read({car,X}),
        Nc = Xcars#car.quantity,
        Leftcars = Xcars#car{quantity = Nc - Ncars},

        %% ---now we update the database
        mnesia:write(Leftcars),

    end,
    mnesia:transaction(F).

请帮助我如何编写从商店购买汽车的功能.

Answer 1

但是你的实现工作正常,除了你在mnesia:write(Leftcars)之后添加非法逗号.这是有效的代码(我尝试将您的实现作为buy_car2).

-module(q).

-export([setup/0, buy_car/2, buy_car2/2]).

-record(car, {brand, quantity}).

setup() ->
    mnesia:start(),
    mnesia:create_table(car, [{attributes, record_info(fields, car)}]),
    mnesia:transaction(fun() -> mnesia:write(#car{brand=bmw, quantity=1000}) end).

buy_car(Brand, Ncars) ->
    F = fun() ->
         [Car] = mnesia:read(car, Brand), % crash if the car is missing
         mnesia:write(Car#car{quantity = Car#car.quantity - Ncars})
    end,
    mnesia:transaction(F).

buy_car2(X,Ncars) ->
    F = fun() ->
        %% ----first i find the number of car X available in the shop
        [Xcars] = mnesia:read({car,X}),
        Nc = Xcars#car.quantity,
        Leftcars = Xcars#car{quantity = Nc - Ncars},
        %% ---now we update the database
        mnesia:write(Leftcars)
    end,
    mnesia:transaction(F).