How to update a Mnesia table in Erlang - database

Question

I have a little problem with my code. I have a table containing car details, name, price and quantity, so I am trying to create a function called buy which will be used to buy a specific car. When a user buys eg 5 BMW cars, they will call buy_car(bmw,5). Now after this I want to update the new value of quantity for BMW cars.
My attempt is below but I can't seem to work around it, I am new to Erlang.
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).
Please help me with how I can write a function that buys cars from the shop.

Answer 1

But your implementation works fine except you added illegal comma after mnesia:write(Leftcars).
Here is code that works (I tried your implementation as 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

I would do something like below:
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).
To use the above method, mnesia table must be created as "ram_copies" and with no replication nodes. Also, if there are lot of updates happening, you might not want to copy the ram_copies to disk for every update due to performance issues, rather you will do it in time triggered manner.