How to pull embedded docs from MongoDB query into array?

Question

I have a variable var correctAnswers;

In my MongoDB I have the following document (below). I am trying to write a query that takes all of the "correct" fields from the "quiz" field and put them into their own array, so I can set that array equal to var correctAnswers;.

"title" : "Economics questions"
"quiz": "[{
          "question": "Which of these involves the analysis of of a business's financial statements, often used in stock valuation?",
          "choices": ["Fundamental analysis", "Technical analysis"],
          "correct": 0
        }, {
          "question": "What was the name of the bond purchasing program started by the U.S. Federal Reserve in response to the 2008 financial crisis?",
          "choices": ["Stimulus Package", "Mercantilism", "Quantitative Easing"],
          "correct": 2
        }, {
          "question": "Which term describes a debt security issued by a government, company, or other entity?",
          "choices": ["Bond", "Stock", "Mutual fund"],
          "correct": 0
        }, {
          "question": "Which of these companies has the largest market capitalization (as of October 2015)?",
          "choices": ["Microsoft", "General Electric", "Apple", "Bank of America"],
          "correct": 2
        }, {
          "question": "Which of these is a measure of the size of an economy?",
          "choices": ["Unemployment rate", "Purchasing power index", "Gross Domestic Product"],
          "correct": 2
        }]"

How should I go about that, or can someone point me in the right direction? I have tried projections, but should I do an aggregation? Thank you for any help.

Edit for clarity: the output I am looking for in this example is an array, [0,2,0,2,2]

what you mean by " "correct" fields from the "quiz" field" ? please define what output you want so that i can help — Himanshu sharma, Sep 13 '16 at 16:29
See here: http://stackoverflow.com/questions/4969768/removing-the-array-element-in-mongodb-based-on-the-position-of-element There isn't a great way of doing this in MongoDB yet and it's been a JIRA item for some time. — dyouberg, Sep 13 '16 at 16:30
@Himanshusharma thank you for your reply, I mean the field that is named "correct" in each object in the array. Edit: the output in this case would be an array, [0,2,0,2,2] — Jake 1986, Sep 13 '16 at 16:36
Provide some code in node.js as you trying to do. Do you think to use Mongoose? — BrTkCa, Sep 13 '16 at 16:41
It seems like you're trying to get an array so you can produce an average? I'm after the same thing and also haven't found a good solution yet. — Nick Schroeder, Sep 14 '16 at 01:44

score 0 · Answer 1 · answered Sep 13 '16 at 16:55

you can get this result [{correct:0},{correct:2},{correct:0},{correct:2}] but [0,2,0,2,2] type of result is not possible unless we use distinct

db.quiz.aggregate( // Initial document match (uses index, if a suitable one is available) { $match: { "title" : "Economics questions" }},
// Convert embedded array into stream of documents
{ $unwind: '$quiz' },

    },

// Note: Could add a `$group` by _id here if multiple matches are expected

// Final projection: exclude fields with 0, include fields with 1
{ $project: {
    _id: 0,
    score: "$quiz.correct"
}} )

score 0 · Answer 2 · answered Sep 13 '16 at 17:00

db.users.find( { }, { "quiz.correct": 1,"_id":0 } )

// above query will return following output :

{
        "quiz" : [
                {
                        "correct" : 0
                },
                {
                        "correct" : 2
                },
                {
                        "correct" : 0
                },
                {
                        "correct" : 2
                },
                {
                        "correct" : 2
                }
        ]
}

Process this output as per requirement in the node js.

RootHacker · Answer 3 · 2016-09-13T18:47:58.690

One way to achieve this through aggregation

db.collectionName.aggregate([
     // use index key in match pipeline,just for e.g using title here
    { $match: { "title" : "Economics questions" }},
    { $unwind: "$quiz" },
    { $group: {
          _id:null,
          quiz: { $push:  "$quiz.correct" }
        } 
    },
    //this is not required, use projection only if you want to exclude/include fields 
    { 
        $project: {_id: 0, quiz: 1}
    } 
])

Above query will give you the following output

{
    "quiz" : [ 0, 2, 0, 2, 2 ]
}

Then simply process this output as per your need.

score 0 · Answer 4 · edited Sep 13 '16 at 18:09

0

Try this:

db.getCollection('quize').aggregate([
{$match:{_id: id }},
{$unwind:'$quiz'}, 
{$group:{
_id:null, 
score: {$push:"$quiz.correct"} 
}}
])

It will give you the expected output.

edited Sep 13 '16 at 18:09

dds

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answered Sep 13 '16 at 17:48

Vaibhav Patil

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How to pull embedded docs from MongoDB query into array?

4 Answers4