Mnesia数据库增加与查询学习

-module(test_mnesia).


-record(shop, {item, quantity, cost}).
-record(cost, {name, price}).
-record(design, {info, plan}).
%% API
-export([insert/3,select/1,start/0]).
start() ->
  mnesia:start().

insert(Name, Quantity, Cost) ->
  Row = #shop{item = Name, quantity = Quantity, cost = Cost},
  F = fun() ->
    mnesia:write(Row)
      end,
  mnesia:transaction(F).

select() ->
  qlc:q([X || X <- mnesia:table(shop)]).

能够插入但是不能查询

需要对do函数进行实现，才能展示出数据库中的数据,do函数在书上20.4.3有相应实现

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

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

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

select() ->
  do(qlc:q([X || X <- mnesia:table(shop)])).

do(Q) ->
  F  = fun
         () ->
           qlc:e(Q)
       end,
  {atomic,Val} = mnesia:transaction(F),
  Val.

实现do函数后的代码运行截图