Aggregation issue with Mongo 3.6

Question

I was using the aggregate function without any problem connecting a 3.4 mongodb.

When I change to a 3.6 db,

I've got the message: The 'cursor' option is required, except for aggregate with the explain argument.

Sorry if it's already posted. I am unable to find any solutions

Answer 1

In mongo 3.6 there has been changes while using aggregate you have to use cursor, unless you include the explain option, you must specify the cursor option. I faced same error as you were facing. Now you have to do it like

this.aggregate( [
        { $unwind : "$tags" },
        {$group: {_id: '$tags', count: { $sum: 1} }},
        {$sort: { count: 1 }}
            ] ).cursor({}).exec();

Now you can use cursor method cursor.toArray() to return an array that contains all the documents from a cursor. Cursors returned from aggregation only supports cursor methods that operate on evaluated cursors like cursor.toArray(), to know about more cursor methods you can click here and get further.

Answer 2

In Node.js, after upgrade MongoDB from 3.4 to 3.6, there are 2 things that need to check:

1. Add cursor: {} option in the aggregate statement. In previous MongoDB version, this is optional (Of course, you can define batchSize in cursor object if you want). For example:

   db.collection(collectionName).aggregate(pipelineArray, {
     cursor: {}
   }, function(error, result) {
     ...
   });
   

2. If the above code hang and the callback is not invoked, please check the mongodb driver version. I get this "not responding" problem in mongodb module v2.2.16. After upgrade mongodb to v2.2.35, it's fixed.

Answer 3

For those with issues trying to figure out a complete example on 3.6, this worked for me :

   db.collection(collectionName).aggregate([
        {
            $lookup: lookUpOption
        },
        {
            $match: matchOption
        }
    ],{cursor: {}},null).toArray(function(err, docs) {
        console.log("returned data is : ", docs);
        db.close();
    });