Mongo DB MapReduce: Emit key from array based on condition
{
    "_id" : ObjectId("4fc5ed3e67960de6794dd21c"),
    "name" : "some name",
    "uid" : "some app specific uid",
    "collection" : "some name",
    "metadata" : [
        {
            "key" : "key1",
            "value" : "Plain text",
            "status" : "SINGLE_RESULT",
        },
        {
            "key" : "key2",
            "value" : "text/plain",
            "status" : "SINGLE_RESULT",
        },
        {
            "key" : "key3",
            "value" : 3469,
            "status" : "OK",
        }
     ]
}

function map() {
   var mime = "";
   this.metadata.forEach(function (m) {
     if (m.key === "key2") {
        mime = m.value;}
     });
     emit(mime, {count:1});
}

function reduce() {
  var res = {count:0};
  values.forEach(function (v) {res.count += v.count;});
  return res;
}

db.collection.mapReduce(map, reduce, {out: { inline : 1}})

{
    "_id" : ObjectId("4fc5ed3e67960de6794dd21c"),
    "name" : "some name",
    "uid" : "some app specific uid",
    "collection" : "some name",
    "metadata" : {
        "key1" : {
            "value" : "Plain text",
            "status" : "SINGLE_RESULT"
        },
        "key2": {
            "value" : "text/plain",
            "status" : "SINGLE_RESULT"
        },
        "key3" : {
            "value" : 3469,
            "status" : "OK"
        }
     }
}

function map() {
   emit( this.metadata["key2"].value, { count : 1 } );
}

function map() {
   var mime = "";
   this.metadata.forEach(function (m) {
     if (m.key === "key2") {
        mime = m.value;
        break;
     }
     });
     emit(mime, {count:1});
}