Using result of one query in another query : nodejs and mongodb

Question

I'm trying to count the number of documents in a collection and then use the result into another insert query using Node.js.

var number;
db.collection("collection").countDocuments({}, function(error, numOfDocs){
    if(error) throw error;
    else{
        console.log(numOfDocs);
        number = numOfDocs;
    }
}); 

var myobj = { num: "Number_"+number };
db.collection("another_collection").insertOne(myobj, function(err, res) 
{
    if (err) throw err;
    console.log("1 document inserted");         
});

The inserted value in the collection is { "num" : "Number_undefined" }

How to use the result of the first query into another?

weirdpanda · Accepted Answer · 2018-09-11 15:15:09Z

2

It is because you have not given Number a value. Further, the count() function is a async function (I wrote an answer here about that). Try this:

db.collection('collection')
  .countDocuments({}, function(error, numOfDocs){
    if(error) throw error;
    else {
        var myobj = { num: 'Number_' + numOfDocs };
        db.collection('another_collection').insertOne(myobj, function(err, res) {
               if (err) throw err;
               console.log('1 document inserted');         
        });
    }
});

answered Sep 11, 2018 at 15:15

weirdpanda

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3 Comments

Akshat Yadav Over a year ago

Thanks @weirdpanda. But this generates an error "MongoError: server instance pool was destroyed". Any idea why this is happening?

weirdpanda Over a year ago

It's not because of this code. Are you using db.close() somewhere after this bit?

Akshat Yadav Over a year ago

Thanks, it worked. I was using db.close() Removed that and it works fine now.

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