Erlang Mnesia Select 结果与 read、foldl 不同

Question

我有 3 个用例 return 从失忆症 table 收集数据 1. return a table 的所有值所以我使用 foldl， 2. return 1 行所以我使用 read 3. return 基于条件的可变数量的记录，所以我使用 select。

我想使用相同的代码来管理结果，但是 select return 的数据结构不同。我希望有人可以帮助我将我的 select 重组为 return，就像其他人一样。

下面是问题的示例代码和结果。问题是 select 不像 read 和 foldl 那样 return table 的记录名称。

-module(testselect2).

-export([runtest/0]).

-record(record_a, {b, c, d}).
-record(record_b, {record_a, e}).
-record(record_c, {record_b, f, intval}).
runtest() ->
mnesia:create_schema([node()]),
mnesia:start(),
mnesia:create_table(record_c, [{attributes, record_info(fields, record_c)}]),

A1 = #record_a{b = "R1", c = "T1", d = "C1"},
B1 = #record_b{record_a = A1, e = "E1"},
C1 = #record_c{record_b = B1, f = "F1", intval = 100},    

A2 = #record_a{b = "R2", c = "T2", d = "C2"},
B2 = #record_b{record_a = A2, e = "E2"},
C2 = #record_c{record_b = B2, f = "F2", intval = 200},    

A3 = #record_a{b = "R3", c = "T3", d = "C3"},
B3 = #record_b{record_a = A3, e = "E3"},
C3 = #record_c{record_b = B3, f = "F3", intval = 300},    

{atomic, Rw} = mnesia:transaction(
              fun () ->
                mnesia:write(C1),
                mnesia:write(C2),
                mnesia:write(C3)                    
              end),


io:fwrite("Result write = ~w~n", [Rw]),   

{atomic, Rr} = mnesia:transaction(
              fun () ->
                      mnesia:read({record_c, B1})

              end),
io:fwrite("Result read = ~w~n", [Rr]),   

{atomic, Rf} =
           mnesia:transaction(fun () ->
                                      mnesia:foldl(fun (Rec, Acc) -> [Rec | Acc] end,
                                                   [],
                                                   record_c)
                              end),
io:fwrite("Result foldl = ~w~n", [Rf]),                                         

MatchHead = #record_c{record_b='', f='',  intval=''},
Guard = {'>', '', 100},
Result = {{'', '', ''}}, 
{atomic, Rs} = mnesia:transaction(
              fun () ->  
                  mnesia:select(record_c, [{MatchHead, [Guard], [Result]}])
              end),
io:fwrite("Result select = ~w~n", [Rs]).

===== 结果

44> testselect2:runtest().

结果写入 = ok

读取结果 = [{record_c,{record_b,{record_a,[82,49],[84,49],[67,49]},[ 69,49]},[70,49],100}]

结果 foldl = [{record_c,{record_b,{record_a,[82,49],[84,49],[67,49]},[ 69,49]},[70,49],100},{record_c,{record_b,{record_a,[82,51],[84,51],[67 ,51]},[69,51]},[70,51],300},{record_c,{record_b,{record_a,[82,50],[84 ,50],[67,50]},[69,50]},[70,50],200}]

结果select = [{{record_b,{record_a,[82,51],[84,51],[67,51]},[69, 51]},[70,51],300},{{record_b,{record_a,[82,50],[84,50],[67,50]},[69, 50]},[70,50],200}]

还行

如您所见，上面的 read 和 foldl 记录以 {record_c,{... 开头，其中 select 缺少 record_c，只有 {{...

我一直无法找到使 select 到 return 相同结构的方法，因此我的处理代码可以适用于所有 3 个用例。任何建议将不胜感激。

Answer 1

我不是记忆专家，但我知道当你使用 ETS 匹配表达式时，你决定了结果是什么样子。您使用 Result = {{'', '', ''}} 来创建结果项，这使得它们在一个元组中以三元组的形式出现，正如我们在您的输出中看到的那样。根据 ets:select/1，您想使用特殊变量 '$_' 来 return 整个匹配对象，因此这应该可以代替您的 Result = ... 行：

Result = '$_',

Erlang Mnesia Select 结果与 read、foldl 不同

Erlang Mnesia Select result not same as read, foldl

erlang

mnesia

还行