Why does MapReduce return undefined for a field that exists in document? - mongodb

Question

I am trying to debug a strange issue I am running into while running mapreduce on a collection:
For reference, here's a single document from the collection:
{
"_id" : "ITOUXFWgvWs",
"source" : "youtube",
"insert_datetime" : ISODate("2017-04-06T22:27:43.598Z"),
"processed" : false,
"raw" : {
"id" : "ITOUXFWgvWs",
"etag" : "\"m2yskBQFythfE4irbTIeOgYYfBU/hiQtS6aptLlqxTpsYp1EJIRcoZo\"",
"snippet" : {
"publishedAt" : ISODate("2017-04-06T13:25:28Z"),
"title" : "Alarm.com: The Only Smart Home App You Need",
"channelId" : "UC_HZfoZUP36STk7SrtKYH4g",
"description" : "All these new connected devices are awesome, but wouldn’t it be great if you could use one app for the entire connected home? It can all come together with Alarm.com.",
"categoryId" : "28",
"channelTitle" : "Alarm.com",
"thumbnails" : {
"default" : {
"height" : 90,
"width" : 120,
"url" : "https://i.ytimg.com/vi/ITOUXFWgvWs/default.jpg"
},
"standard" : {
"height" : 480,
"width" : 640,
"url" : "https://i.ytimg.com/vi/ITOUXFWgvWs/sddefault.jpg"
},
"high" : {
"height" : 360,
"width" : 480,
"url" : "https://i.ytimg.com/vi/ITOUXFWgvWs/hqdefault.jpg"
},
"medium" : {
"height" : 180,
"width" : 320,
"url" : "https://i.ytimg.com/vi/ITOUXFWgvWs/mqdefault.jpg"
},
"maxres" : {
"height" : 720,
"width" : 1280,
"url" : "https://i.ytimg.com/vi/ITOUXFWgvWs/maxresdefault.jpg"
}
},
"liveBroadcastContent" : "none",
"localized" : {
"title" : "Alarm.com: The Only Smart Home App You Need",
"description" : "All these new connected devices are awesome, but wouldn’t it be great if you could use one app for the entire connected home? It can all come together with Alarm.com."
}
},
"contentDetails" : {
"duration" : "PT37S",
"dimension" : "2d",
"definition" : "hd",
"licensedContent" : false,
"projection" : "rectangular",
"caption" : "false"
},
"kind" : "youtube#video",
"statistics" : {
"likeCount" : "0",
"dislikeCount" : "0",
"favoriteCount" : "0",
"viewCount" : "32"
},
"uploaded" : ISODate("2017-04-06T13:25:28Z")
},
}
I am literally following the mapreduce debug steps from official mongo documentation.
Here's what my mapreduce script looks like:
var map = function() {
emit("1", this._id);
};
var emit = function(key, value) {
print("emit");
print("key: " + key + " value: " + tojson(value));
}
var myDoc = db.getCollection("abc").find({}).limit(1);
map.apply(myDoc);
And it always produces the result like this:
MongoDB shell version: 2.4.6
connecting to: test
emit
key: 1 value: undefined
I expect the script to emit the _id since it clearly exists in the document but it doesn't.
What might be possible cause of this?

Answer 1

find() always returns a cursor.
Replace it with findOne()
var myDoc = db.getCollection("abc").findOne({});
Or store the documents in an Array using toArray()
var myDoc = db.getCollection("abc").find({}).limit(1).toArray()[0];