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我正在尝试制作 MongooseIM 模块，该模块将使用 offline_message_hook 触发，为用户计算离线存储中待处理消息的数量，并通过 GET 方法将其发送到 URL。下面是我的代码。

send_notice(From, To, Packet) ->
    Type = xml:get_tag_attr_s(list_to_binary("type"), Packet),
    Body = xml:get_path_s(Packet, [{elem, list_to_binary("body")}, cdata]),
    PostUrl = "http://myurl.com/",
    Count = count_msg(To),
    GetParam = "?count=",
    FullUrl = PostUrl ++ GetParam ++ Count,

    ?DEBUG("POST URL : ~s",[FullUrl]),

    if (Type == <<"chat">>) and (Body /= <<"">>) ->
              Sep = "&",
        Post = [
          "alert=", url_encode(binary_to_list(Body)), Sep,
                                        "badge=", url_encode("+1"), Sep,
          % "sound=", Sound, Sep,
          "channel=", To#jid.luser, Sep,
          "info[from]=", From#jid.luser, Sep],
          % "auth_token=", Token],

        ?INFO_MSG("Sending post request to ~s with body \"~s\"", [FullUrl, Post]),

        httpc:request(post, {binary_to_list(FullUrl), [], "application/x-www-form-urlencoded", list_to_binary(Post)},[],[]),
        ok;
      true ->
        ok
    end.

count_msg(To) ->
     Username = To#jid.luser,
     LServer = To#jid.lserver,
     Count = ejabberd_odbc:sql_query(
      LServer,
      ["select count(*) from offline_message "
       "where username='", Username, "';"]),
     ?DEBUG("Count = ~s",[Count]),
     Count.

当我运行它时，我收到以下错误

2015-03-09 16:37:11.598 [debug] <0.763.0>@mod_zeropush:count_msg:102 FORMAT ERROR: "Count = ~s" [{selected,[<<"count">>],[{<<"5">>}]}]
["select count(*) from offline_message where username='",<<"reader">>,"';"]
2015-03-09 16:37:11.599 [debug] <0.763.0>@mod_zeropush:send_notice:72 FORMAT ERROR: "POST URL : ~s" [[104,116,116,112,58,47,47,107,107,104,97,110,46,100,108,99,119,111,114,108,100,119,105,100,101,46,99,111,109,47,63,99,111,117,110,116,61|{selected,[<<"count">>],[{<<"5">>}]}]]
2015-03-09 16:37:11.599 [info] <0.763.0>@mod_zeropush:send_notice:84 FORMAT ERROR: "Sending post request to ~s with body \"~s\"" [[104,116,116,112,58,47,47,107,107,104,97,110,46,100,108,99,119,111,114,108,100,119,105,100,101,46,99,111,109,47,63,99,111,117,110,116,61|{selected,[<<"count">>],[{<<"5">>}]}],["alert=","xxx","&","badge=","%2B1","&","channel=",<<"reader">>,"&","info[from]=",<<"kkhan">>,"&"]]
2015-03-09 16:37:11.602 [error] <0.763.0>@ejabberd_hooks:run1:240 {badarg,[{erlang,binary_to_list,[[104,116,116,112,58,47,47,107,107,104,97,110,46,100,108,99,119,111,114,108,100,119,105,100,101,46,99,111,109,47,63,99,111,117,110,116,61|{selected,[<<"count">>],[{<<"5">>}]}]],[]},{mod_zeropush,send_notice,3,[{file,"src/mod_zeropush.erl"},{line,86}]},{safely,apply,3,[{file,"src/safely.erl"},{line,19}]},{ejabberd_hooks,run1,3,[{file,"src/ejabberd_hooks.erl"},{line,236}]},{ejabberd_sm,route,3,[{file,"src/ejabberd_sm.erl"},{line,108}]},{ejabberd_local,route,3,[{file,"src/ejabberd_local.erl"},{line,139}]},{ejabberd_router,route,3,[{file,"src/ejabberd_router.erl"},{line,78}]},{ejabberd_c2s,session_established2,2,[{file,"src/ejabberd_c2s.erl"},{line,1098}]}]}

在erlang中获取SQL查询结果的正确方法是什么？httpc:request 期望什么格式？

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1 回答 1

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您得到的错误并不完全是 SQL 或 MongooseIM 特定的错误。它们是错误使用io_lib:format/2或类似的字符串格式化函数。

这个错误：

2015-03-09 16:37:11.598 [debug] <0.763.0>@mod_zeropush:count_msg:102 FORMAT ERROR: "Count = ~s" [{selected,[<<"count">>],[{<<"5">>}]}]

涉及到：

?DEBUG("Count = ~s", [Count])

但是Count既不是字符串，也不是二进制，也不是 iolist，也不是原子（只有这些类型可以使用~s- 字符串格式化字符打印；请参阅man io函数format/3）。要使用默认 Erlang 术语语法打印复杂术语，请使用~p或~w。

您所有的FORMAT ERROR错误都属于这种类型 - 您为传入的数据类型使用了错误的格式说明符。

好的，那么 SQL 查询结果的格式是什么？它打印在错误行之一中：

{selected, [<<"count">>], [{<<"5">>}]}

这是您在 SQL shell 中获得的内容的简单音译（此示例来自我手头在 PostgreSQL 中的一个随机表）：

> SELECT count(*) FROM conversation_state;
 count
-------
     2
(1 row)

这是从 Erlang shell 运行的等效程序：

> ejabberd_odbc:sql_query(<<"localhost">>, ["SELECT count(*) FROM conversation_state;"]).
{selected,[<<"count">>],[{<<"2">>}]}

所以结果总是一个三元组；第一个元素是selected（或inserted，updated...取决于查询）；第二个元素是列标题列表（这里只有一列count）；第三个元素是一个元组列表，其中每个元组对应于结果集中的一行。rows-as-tuples 的元素与列标题的顺序相同。

关于最后一个错误：

2015-03-09 16:37:11.602 [error] <0.763.0>@ejabberd_hooks:run1:240 {badarg,[{erlang,binary_to_list,[[104,116,116,112,58,47,47,107,107,104,97,110,46,100,108,99,119,111,114,108,100,119,105,100,101,46,99,111,109,47,63,99,111,117,110,116,61|{selected,[<<"count">>],[{<<"5">>}]}]],[]},{mod_zeropush,send_notice,3,[{file,"src/mod_zeropush.erl"},{line,86}]},{safely,apply,3,[{file,"src/safely.erl"},{line,19}]},{ejabberd_hooks,run1,3,[{file,"src/ejabberd_hooks.erl"},{line,236}]},{ejabberd_sm,route,3,[{file,"src/ejabberd_sm.erl"},{line,108}]},{ejabberd_local,route,3,[{file,"src/ejabberd_local.erl"},{line,139}]},{ejabberd_router,route,3,[{file,"src/ejabberd_router.erl"},{line,78}]},{ejabberd_c2s,session_established2,2,[{file,"src/ejabberd_c2s.erl"},{line,1098}]}]}

您正在尝试传递使用FullUrl = PostUrl ++ GetParam ++ Countto创建的列表erlang:binary_to_list/1。顾名思义，这个函数接受二进制文件，而不是列表。Erlang 是动态的，但是是强类型的，并且几乎没有隐式类型转换。正确解压缩 SQL 查询的结果，只返回其中的数字count_msg并从中构建 HTTP 请求 URL。

于 2015-03-10T10:05:05.540 回答