Emit Tuples From Erlang Views In CouchDB

Question

CouchDB, version 0.10.0, using native erlang views.

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"
}

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});
            });
          }
       }"
     }
   }
}

which emits:

{"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"}}
]}

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.

which emits:

{"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"]}
]}

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);

Fails with the following error:

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

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.

Answer 1

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

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

It's not pretty, but very efficient :)

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

1. Start CouchDB with the -i (interactive) flag
2. On the resulting erlang shell, type: couch_util:json_decode(<<"{\"foo\":\"bar\"}">>).
3. Profit

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

Answer 2

For test_suite_reports bd, that has tests field:

[
   {
       "name": "basics",
       "status": "success",
       "duration": 21795
   },
   {
       "name": "all_docs",
       "status": "success",
       "duration": 385
   } ...

I have wrote this to get name and status:

fun({Doc}) ->
  Name = fun(L) ->  proplists:get_value(<<"name">>, L, null) end,
  Status = fun(L) -> proplists:get_value(<<"status">>, L, null) end,
  Tests = proplists:get_value(<<"tests">>, Doc, null),
  lists:foreach(fun({L}) -> Emit(Name(L), Status(L)) end, Tests)
end.

Answer 3

If you like experimental features (that still work...), you might want to have a look to Erlang exprecs.

I found it extremely helpful in creating a sort of dynamic records for Erlang.