在 CouchDB 中从 Erlang 视图中发出元组 [英] Emit Tuples From Erlang Views In CouchDB

问题描述

CouchDB，版本 0.10.0，使用原生 erlang 视图.

CouchDB, version 0.10.0, using native erlang views.

我有一个简单的表格文件:

I have a simple document of the form:

{
   "_id": "user-1",
   "_rev": "1-9ccf63b66b62d15d75daa211c5a7fb0d",
   "type": "user",
   "identifiers": [
       "ABC",
       "DEF",
       "123"
   ],
   "username": "monkey",
   "name": "Monkey Man"
}

还有一份基本的 javascript 设计文档:

And a basic javascript design document:

{
   "_id": "_design/user",
   "_rev": "1-94bd8a0dbce5e2efd699d17acea1db0b",
   "language": "javascript",
   "views": {
     "find_by_identifier": {
       "map": "function(doc) {
          if (doc.type == 'user') {
            doc.identifiers.forEach(function(identifier) {
              emit(identifier, {"username":doc.username,"name":doc.name});
            });
          }
       }"
     }
   }
}

发出:

{"total_rows":3,"offset":0,"rows":[
{"id":"user-1","key":"ABC","value":{"username":"monkey","name":"Monkey Man"}},
{"id":"user-1","key":"DEF","value":{"username":"monkey","name":"Monkey Man"}},
{"id":"user-1","key":"123","value":{"username":"monkey","name":"Monkey Man"}}
]}

我正在研究建立一个做同样事情的 Erlang 视图.迄今为止最好的尝试是:

I'm looking into building an Erlang view that does the same thing. Best attempt so far is:

%% Map Function
fun({Doc}) ->
    case proplists:get_value(<<"type">>, Doc) of
    undefined ->
        ok;
    Type ->
        Identifiers = proplists:get_value(<<"identifiers">>, Doc),
        ID = proplists:get_value(<<"_id">>, Doc),
        Username = proplists:get_value(<<"username">>, Doc),
        Name = proplists:get_value(<<"name">>, Doc),
        lists:foreach(fun(Identifier) -> Emit(Identifier, [ID, Username, Name]) end, Identifiers);
    _ ->
        ok
    end
end.

发出:

{"total_rows":3,"offset":0,"rows":[
{"id":"user-1","key":"ABC","value":["monkey","Monkey Man"]},
{"id":"user-1","key":"DEF","value":["monkey","Monkey Man"]},
{"id":"user-1","key":"123","value":["monkey","Monkey Man"]}
]}

问题是 - 我怎样才能将这些值作为元组而不是数组取出?我不认为我可以(或不想)使用记录，但在元组中使用原子似乎不起作用.

The question is - how can I get those values out as tuples, instead of as arrays? I don't imagine I can (or would want to) use records, but using atoms in a tuple doesn't seem to work.

lists:foreach(fun(Identifier) -> Emit(Identifier, {id, ID, username, Username, name, Name}) end, Identifiers);

失败并出现以下错误:

{"error":"json_encode","reason":"{bad_term,{<<"user-1">>,<<"monkey">>,<<"Monkey Man">>}}"}

想法?我知道 Erlang 不适合这种特定类型的事情(命名访问)，而且我可以按照约定来做(id 在第一个位置，用户名在下，真名在后)，但这使得客户端代码非常难看.

Thoughts? I know that Erlang sucks for this specific kind of thing (named access) and that I can do it by convention (id at first position, username next, real name last), but that makes the client side code pretty ugly.

推荐答案

JSON 对象 {"foo":"bar","baz":1} 是 {[{<<"foo">>,<<"bar">>},{<<"baz">>,1}]}

The JSON object {"foo":"bar","baz":1} is {[{<<"foo">>,<<"bar">>},{<<"baz">>,1}]}

在 Erlang lingua 中，它是一个包裹在元组中的 proplist.

In Erlang lingua it is a proplist wrapped in a tuple.

它不漂亮，但非常有效:)

It's not pretty, but very efficient :)

为了感受它，您可以使用 CouchDB 附带的 JSON 库:

To get a feel for it you can play with the JSON lib that ships with CouchDB:

1. 使用 -i 启动 CouchDB(交互式)标志
2. 在生成的 erlang shell 中，输入:couch_util:json_decode(<<"{"foo":"bar"}">>).
3. 利润

//在 CouchDB 的更高版本中，这是 ejson:decode()

// in later versions of CouchDB, this is ejson:decode()

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