erlang学习: Mnesia Erlang数据库3

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#数据库 #erlang #学习

Mnesia数据库删除实现和事务处理

-module(test_mnesia).
-include_lib("stdlib/include/qlc.hrl").

-record(shop, {item, quantity, cost}).
%% API
-export([insert/3, select/0, select/1, delete/1, transaction/1,start/0, do_this_once/0]).
start() ->
  mnesia:start().

insert(Name, Quantity, Cost) ->
  Row = #shop{item = Name, quantity = Quantity, cost = Cost},
  io:format("insert ~p~n", [Row]),
  F = fun() ->
    mnesia:write(Row)
      end,
  mnesia:transaction(F).

delete(Item) ->
  Oid = {shop, Item},
  io:format("delete ~p~n", [Oid]),
  F = fun() ->
    mnesia:delete(Oid)
      end,
  mnesia:transaction(F).

select() ->
  do(qlc:q([X || X <- mnesia:table(shop)])).
select(Num) ->
  do(qlc:q([X || X <- mnesia:table(shop), X#shop.quantity =< Num])).
%%事务函数
transaction(Num) ->
  F = fun
        () ->
          [Apple] = mnesia:read({shop, apple}),
          NApple = Apple#shop.quantity,
          Apple1 = Apple#shop{quantity = NApple + Num},
          mnesia:write(Apple1),
          [Orange] = mnesia:read({shop, orange}),
          NOrange = Orange#shop.quantity,
          if
            NOrange >= Num ->
              N1 = NOrange - Num,
              Orange1 = Orange#shop{quantity = N1},
              mnesia:write(Orange1);
            true ->
              mnesia:abort(oranges)
          end
      end,
  mnesia:transaction(F).
do(Q) ->
  F = fun
        () ->
          qlc:e(Q)
      end,
  {atomic, Val} = mnesia:transaction(F),
  Val.


do_this_once() ->
  mnesia:create_schema([node()]),
  mnesia:start(),
  mnesia:create_table(shop, [{attributes, record_info(fields, shop)}]),
  mnesia:stop().