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，因此我尝试创建一个名为buy的功能，用于购买特定的汽车。当用户购买5辆宝马汽车时，他们会拨打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（我尝试将你的实现作为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).

Answer 2

我会做类似下面的事情：

Considering the record is defined as :
-record(car_record, {car, quantity}).

The following function will update the data:
buy_car(X,NCars) ->
    Row = #car_record{car = X, quantity = NCars}.
    mnesia:ets(fun()-> mnesia:dirty_write(Row) end),
    mnesia:change_table_copy_type(guiding_data, node(), disc_copies).

要使用上述方法，必须将mnesia表创建为“ram_copies”且没有复制节点。此外，如果发生了大量更新，您可能不希望因性能问题而将ram_copies复制到磁盘以进行每次更新，而是以时间触发的方式执行此操作。

如何在Erlang中更新Mnesia表

2 个答案: