database - 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，因此我试图创建一个名为 buy 的函数，用于购买特定汽车。例如，当用户购买 5 辆 BMW 汽车时，他们将调用 buy_car(bmw,5)。在此之后，我想更新 BMW 汽车数量的新值。

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).

请帮助我编写一个从商店购买汽车的函数。

Answer 1

但是你的实现工作正常，除了你在 mnesia:write(Leftcars) 之后添加了非法逗号。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).