How to find the documents which have an empty array in them in MongoDB using Mongoose in Node.JS?

Question

In the 'QAns' model, I want to load all the documents in which the 'Answers' field is an empty array using mongoose in node.js. I am using MongoDB Atlas for storing my data. This is my schema for the database:

const QuestAnsSchema = new Schema({
    text: String,
    author: String,
    answers: [{
        text: String,
        author: String
   }]
});

This is the data in the MongoDB database.

{
     "_id":{"$oid":"5ee1f235a1e9870daca7d5e9"},
     "text":"Question 1",
     "author":"5ee1b8ebdbf91b23a808d417",
     "answers":[],
     "__v":{"$numberInt":"0"}
},
{
     "_id":{"$oid":"5ee1f235885770darrr7f449"},
     "text":"Question 2",
     "author":"5ee1b8ebdbf9o2w3a808d417",
     "answers":[],
     "__v":{"$numberInt":"0"}
}

Both the documents have the 'answers' field empty in them but assuming there would be some documents with a non-empty 'answers' field how will I load the ones with no answers field?

I have tried this code but it gives a 'Bad Request' error:

router.get('/questions', (req, res) => {
    QAns.find({answers: { $exists: true, $ne: [] } }).then(( err, result) => {
        if(err)  return res.sendStatus(400);

        res.render('questions',{ result });

        console.log(result);
    });

})

QAns is the model name, answers is the array field
'questions' is the ejs file which will display those documents after they are loaded.

Yousaf · Accepted Answer · 2020-06-12 10:23:35Z

6

You can use $size operator to fetch all the documents where answers array is of length zero.

QAns.find({
  answers: { $size: 0 } 
})

$size operator is used to query an array by number of elements.

Edit:

To select all the documents where answers array is not empty, use $not operator along with $size operator.

Following query will select all the documents where answers array is not empty

QAns.find({
  answers: {
    $not: { $size: 0 }
  }
})

$not operator performs a logical NOT operation on the specified <operator-expression> and selects the documents that do not match the <operator-expression>

P.S. Above query will also return those documents where answers field is not present

edited Jun 12, 2020 at 10:23

answered Jun 11, 2020 at 11:40

Yousaf

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2 Comments

Kunal Nk Over a year ago

Thanks, it worked! How do I do this for a non-empty answers field?

Yousaf Over a year ago

you mean you want to find those documents where answers array is not empty? If yes then see the updated answer

Tom Slabbaert · Accepted Answer · 2020-06-11 12:00:20Z

0

$exists works great for your needs, you just have a syntax error in your query coming from your $ne.

It's unclear if you want documents with answers or without so ill provide both options:

this will provide documents without the answer field:

QAns.find({answers: { $exists: false })

this will provice documents with at least one answer in the answers field:

QAns.find({"answers.0": { $exists: true } })

this will provide documents with answers=[]

QAns.find({"optOuts.0": {$exists: false}, optOuts: {$exists: true}})

Or as Yousaf suggjested:

QAns.find({"answers": { $size: 0 } })

edited Jun 11, 2020 at 12:00

answered Jun 11, 2020 at 11:45

Tom Slabbaert

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3 Comments

Yousaf Over a year ago

OP wants to fetch documents which have empty answers array, not those documents where answers field doesn't exists or those where there's atleast one item in answers array.

Tom Slabbaert Over a year ago

I'm not so sure you're right as the explanation is poorly worded and the code snipper he posted has $ne: [].

Yousaf Over a year ago

I want to load all the documents in which the 'Answers' field is an empty array

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