包含从erlang中的两个表读取和写入的函数答案

【问题标题】：function which contains read and write from two table in erlang包含从erlang中的两个表读取和写入的函数
【发布时间】：2013-02-18 23:27:11
【问题描述】：

我在 mnesia 中有两个表格，格式如下：

mnesia:create_table(person,
   [{disc_copies, [node()]},
    {attributes, record_info(fields, person)}]),
mnesia:create_table(person_backup,
   [{disc_copies, [node()]},
    {attributes, record_info(fields, person)},
    {record_name, person}]),

我想开发一个具有此作用的功能：

从表person中读取所有数据然后将这些数据写入表 person_backup 中

我尝试使用备份功能（这是你的代码）

testbackup()->

    mnesia:transaction(fun() ->
  Records = mnesia:select(person, [{'_', [], ['$_']}]),
  [ok = mnesia:write(person_backup, Record, write) || Record <- Records]
end).

当我运行这个函数时，我有这个消息

 model:testbackup().
{atomic,[ok,ok,ok,ok,ok,ok,ok,ok,ok,ok,ok,ok,ok,ok,ok,ok,ok,
         ok,ok,ok,ok,ok,ok,ok,ok,ok,ok|...]}

它完美地工作意味着在表 person_backup 中我拥有与表 person 相同的数据

但是当我做模型时：reset().

将删除person表和person_backup表的数据

通常只是删除人的数据

reset() 的代码是：

reset() ->
    stop(),
    destroy(),
    create(),
    start(),

    {ok}.


destroy() ->
    mnesia:start(),
    mnesia:delete_table(counter),
    mnesia:delete_table(person),
    mnesia:stop(),
    mnesia:delete_schema([node()]).


create() ->
    mnesia:create_schema([node()]),
    mnesia:start(),
    mnesia:create_table(counter, [{attributes, record_info(fields, counter)}, {disc_copies, [node()]}]),
    mnesia:create_table(person, [{attributes, record_info(fields, person)}, {disc_copies, [node()]}]),
  mnesia:create_table(person_backup,[{disc_copies, [node()]},{attributes, record_info(fields, person)},
    {record_name, person}]),
    mnesia:stop().

我尝试用另一种方式解决这个问题

我有一个函数testcreate

testcreate()->

    %% mnesia:create_schema([node()]),
    mnesia:start(),

    mnesia:create_table(person_backup, [{attributes, record_info(fields, person_backup)}, {disc_copies, [node()]}]).

在我的记录中

-record(person, {id, token, password, pin, key, salt, pin_salt, subscription_date, first_name, last_name, alias, gender, status,
                 taxid, formid, idcard, birth_year, birth_month, birth_date}).
-record(person_backup, {id, token, password, pin, key, salt, pin_salt, subscription_date, first_name, last_name, alias, gender, status,
                 taxid, formid, idcard, birth_year, birth_month, birth_date}).

当我运行函数 testbackup 我有这个消息

 2> model:testbackup().
    {aborted,{bad_type,{person,215,"97808233",
                               "bddcba13effb029e93aaab6fdc3c4587",
                               "d707aa5f940a468e149686b3eaafd946",
                               "230d8294d47f6fa2cc1761deab52a879",
                               "1360713353326653","1360713353326653",undefined,
                               "souad","sallami",[],"M.","preregistered",
                               undefined,"Z008022","04705808","CIN",[],
                               "d41d8cd98f00b204e9800998ecf8427e","0","user",0,
                               {{...},...},
                               undefined,...}}}

【问题讨论】：

标签： erlang

【解决方案1】：

类似这样的：

mnesia:transaction(fun() ->
  Records = mnesia:select(person, [{'_', [], ['$_']}]),
  [ok = mnesia:write(person_backup, Record, write) || Record <- Records]
end).

【讨论】：