"bad_type" 尝试创建 mnesia 表时出错

Question

我这辈子都弄不明白。我正在尝试创建 mnesia 表，但不断收到这个奇怪的错误。

这是我的命令：

ok = mnesia:create_schema(Nodes),
rpc:multicall(Nodes, application, start, [mnesia]),
{_, ok} = mnesia:create_table(rr_events,
        [{attributes, record_info(fields, rr_events)},
         {index, [#rr_events.key]},
         {disc_copies, Nodes}]),
rpc:multicall(Nodes, application, stop, [mnesia]).

这是我的记录：

-record(rr_events, {key, events=[]}).

这是错误：

=PROGRESS REPORT==== 24-Mar-2016::21:53:42 ===
         application: mnesia
          started_at: nonode@nohost
** exception error: no match of right hand side value
                    {aborted,{bad_type,rr_events,{index,[2]}}}
     in function  rr:install/1 (c:/Users/zzzz/Projects/zzz/zzz/rr/rr/_build/default/lib/rr/src/rr.erl, line 13)

知道这可能是什么吗？想不通。

Answer 1

呸！感谢这个 excellent blog post 引导我找到答案，引用：

This error:

{aborted,{bad_type,wrud_record,{index,[2]}}}

will occur if you used the first element of the record to index one table, like:

-record(wrud_record, {user, date, label, remark, url}).

and

mnesia:create_table( wrud_record,[ {index,[user]}, {attributes, record_info(fields, wrud_record)}])

, so you should change the index to another element like remark here:

mnesia:create_table( wrud_record,[ {index,[remark]}, {attributes, record_info(fields, wrud_record)}])

everything will be fine. :)

Answer 2

我最近遇到了这个问题。 Learn you some Erlang 声明如下：

Note that you do not need to put an index on the first field of the record, as this is done for you by default.

如果您只需要在记录的第一个元素上建立索引，那么我建议您省略 {index, [record_name]}。

此外，虽然 LYSE 的段落暗示它 official Erlang documentation 更进一步并指出：

index. This is a list of attribute names, or integers, which specify the tuple positions on which Mnesia is to build and maintain an extra index table.

"bad_type" 尝试创建 mnesia 表时出错

"bad_type" error when trying to create mnesia tables

erlang

mnesia